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I  i — h-  i  A 


ELEMENTARY 


METEOROLOGY 


BY 

WILLIAM   MORRIS   DAVIS 

PROFESSOR    OF    PHYSICAL    GEOGRAPHY    IX    HARVARD    COLLEGE 


GINN  &  COMPANY 

BOSTON  •  NEW  YORK  •  CHICAGO  •  LONDON 


COPYRIGHT,  1894, 

BY  WILLIAM  MORRIS  DAVIS. 


ALL  RIGHTS  RESERVED. 
210.2 


fltfrenatum 


(,1NN   &   COMPANY  •  PRO- 
PRIETORS •  BOSTON  •  U.S.A. 


PREFACE. 


THOSE  who  are  already  acquainted  with  the  science  of  meteorology  will 
need  no  words  of  mine  to  show  that  the  greatest  share  of  whatever  value  this 
book  may  contain  comes  from  my  having  studied  and  followed  the  work  of  the 
late  Professor  William  Ferrel  of  Washington.  To  his  remarkable  insight  and 
ingenious  analysis  we  owe  the  best  part  of  the  understanding  of  general 
atmospheric  processes  that  has  yet  been  reached.  Those  who  here  first  come 
to  know  something  of  the  science  of  the  atmosphere,  and  who  are  perhaps  thus 
brought  to  desire  further  acquaintance  with  it,  should  not  fail  to  study  Ferrel's 
Popular  Treatise  on  the  Winds,  in  which  his  more  mathematical  essays  are 
reduced  to  a  simpler  form. 

Yet  after  Ferrel,  an  almost  equal  indebtedness  must  be  acknowledged  to 
Professor  Julius  Hann  of  Vienna,  from  whose  broad  and  accurate  studies  I 
have  found  assistance  at  every  turn.  His  Klimatoloyie,  the  standard  work  of 
the  kind,  and  his  numerous  special  articles,  have  furnished  me  with  much 
information  as  well  as  with  many  well-defined  examples  of  meteorological 
conditions  and  processes. 

The  names  of  many  other  meteorologists  to  whom  acknowledgment  is  due 
come  to  mind  while  revising  the  pages  of  this  book,  which  presents  the 
condensed  results  of  my  reading,  observing  and  teaching  during  the  last  fifteen 
years.  Some  of  these  names  are  mentioned  on  the  appropriate  pages,  but  in 
general  I  have  not  attempted  to  make  explicit  reference  to  the  sources  of 
information  that  have  been  consulted,  believing  that  in  a  school  book,  as  this 
is  primarily  intended  to  be,  such  references  are  not  of  much  value,  especially 
as  they  too  commonly  lead  to  sources  of  information  inaccessible  in  school 
libraries.  A  personal  acknowledgment  must,  however,  be  made  here  to  Mr. 
H.  H.  Clayton,  of  Blue  Hill  Observatory,  for  assistance  in  connection  with 
the  chapter  on  clouds  ;  to  Mr.  Alexander  McAdie,  of  the  Weather  Bureau  in 
Washington,  for  his  advice  in  the  sections  on  atmospheric  electricity  ;  and 
to  Mr.  R.  DeC.  Ward,  successor  of  Professor  Harrington  as  editor  of  the 
American  Meteorological  Journal,  for  many  suggestions  in  connection  with  the 
teaching  of  meteorology,  in  which  he  has  for  some  years  been  associated  with 
me  in  Harvard  College. 


251126 


IV  PREFACE. 

There  are  certain  books  to  which  the  teacher  and  the  independent  student 
should,  if  possible,  have  access.  Besides  the  works  by  Ferrel  and  Hann, 
above  named,  the  list  should  include  Abercromby's  Weather  (New  York,  1887), 
Blanford's  Climates  and  Weather  of  India  (London,  1889),  Buchan's  article  on 
Meteorology  in  the  ninth  edition  of  the  Encyclopedia,  Britannica,  and  his  essay 
on  Atmospheric  Circulation,  with  its  elaborate  series  of  isothermal  and  isobaric 
charts  for  every  month  in  the  year,  in  a  volume  of  the  report  on  the  Challenger 
Expedition  (London,  1889  —  unfortunately,  only  a  small  edition  of  this  great 
work  has  been  published,  and  its  cost  is  comparatively  high),  Eliot's  Handbook 
of  Cyclones  in  the  Bay  of  Bengal  (Calcutta,  1890),  Greely's  American  Weather 
(New  York,  1888),  Scott's  Elementary  Meteorology  (London,  1885),  Sprung's 
Lehrbuch  der  Meteorologie  (Hamburg,  1885  —  the  best  elementary  mathematical 
statement  of  the  subject),  and  Waldo's  Modern  Meteorology  (New  York,  1893). 
The  meteorological  section  of  Berghaus'  Physische  Atlas  (Gotha,  1887),  will 
be  found  of  much  assistance.  Abbe's  translation  of  foreign  meteorological 
memoirs,  recently  published  by  the  Smithsonian  Institution,  will  be  useful 
to  those  who  are  proficient  in  mathematical  physics,  as  indicating  the 
direction  of  advanced  research  in  meteorology  by  the  best  European  investi- 
gators. 

The  current  progress  of  meteorology  can  be  best  followed  by  reading  the 
Meteor ologische  Zeitschrift  (Vienna),  which  contains  valuable  original  essays 
and  a  full  bibliographical  review  of  the  science  ;  or  the  American  Meteorological 
Journal  (published  by  Ginn  &  Co.,  Boston),  which,  although  less  extended 
than  the  German  journal,  is  more  serviceable  and  accessible  to  teachers  in  this 
country.  Those  who  intend  to  maintain  regular  meteorological  records  should 
apply  to  their  local  state  weather  service  or  to  the  national  Weather  Bureau 
at  Washington  for  instructions  as  to  the  exposure  and  observation  of  their 
instruments.  Hazen's  Handbook  of  Meteorological  Tables  (Washington,  1888), 
or  the  more  extended  Smithsonian  Meteorological  Tables  (1893)  will  be  found 
useful  in  the  reduction  of  observations. 

It  is  expected  that  students  who  follow  this  book  in  the  later  years  of  a 
high-school  course  or  in  the  earlier  years  of  college  study,  shall  already  have 
had  an  elementary  course  in  physics,  such  as  all  high  schools  should  provide ; 
and  that  they  shall  have  had  in  younger  school-years  a  general  acquaintance 
with  the  facts  of  weather  changes  that  are  illustrated  on  the  daily  weather- 
maps  issued  by  our  national  Weather  Bureau,  as  recently  recommended  by  the 
Conference  on  Geography  of  the  National  Educational  Association  (1893). 
If  these  maps  have  not  been  previously  studied,  they  should  be  secured  by 
application  to  the  central  office  of  the  Weather  Bureau  at  Washington,  or 
from  the  nearest  map  publishing  office,  and  utilized  after  the  manner  described 
in  Section  319.  In  any  case,  the  maps  are  of  much  service  in  aiding  the 


PREFACE.  V 

understanding  of  local  weather  changes  in  their  relation  to  the  more  general 
processes  of  the  atmosphere. 

In  the  use  of  the  book,  the  teacher  should  frequently  direct  the  attention 
of  students  to  the  continuity  of  argument  by  which  the  whole  subject  is  bound 
together,  and  to  the  contrasts  between  inductions  based  on  extended  observa- 
tion, and  deductions  based  on  accepted  physical  laws.  The  latter  as  well  as 
the  former  must  be  carefully  considered  by  those  who  would  gain  an  appre- 
ciation of  the  present  position  of  the  science  and  who  would  derive  the  best 
mental  training  from  its  study.  As  an  aid  in  acquiring  a  general  view  of  the 
subject  in  its  educational  aspects,  teachers  may  refer  to  an  article  by  the 
author  on  "  Meteorology  in  the  Schools,"  in  the  American  Meteorological  Journal 
for  May,  1892. 

It  should  be  noticed  that  the  more  important  general  conceptions,  such  as 
the  arrangement  of  isobaric  surfaces,  or  the  vertical  temperature  gradient,  are 
gradually  introduced ;  at  first  in  their  simplest  form,  and  later  in  greater 
complication.  The  all-important  process  of  convection  is  illustrated  in  local 
examples,  with  particular  definition  of  the  conditions  of  its  occurrence,  in 
Chapter  III  ;  then  in  a  larger  way  for  the  earth  as  a  whole  in  Chapter  VI  ; 
and,  finally,  the  aid  given  to  the  process  by  the  condensation  of  water  vapor  is 
added  in  Chapter  IX.  The  explanation  of  the  general  circulation  of  the 
atmosphere  by  simple  convection  in  Chapter  VI  is  found  to  need  a  supple- 
ment (the  deflective  force  of  the  earth's  rotation)  at  first  unperceived  in  the 
deductive  statement  of  the  problem  ;  and  thus  a  useful  lesson  is  given  in  the 
importance  of  confirming  deductive  explanations  by  continually  confronting 
their  consequences  with  the  facts  of  observation.  This  lesson  is  repeated  in  a 
somewhat  different  form  in  Chapter  X.  On  perceiving  the  similarity  in  the 
arguments  of  these  two  chapters  (Section  230)  much  confidence  may  be  placed 
in  the  amended  convectional  theory  ;  but  another  wholesome  lesson  is  taught 
on  discovering  that  our  extra-tropical  cyclones  are  not  even  yet  certainly 
explained.  A  useful  exercise  in  the  suspension  of  judgment  is  here  gained  in 
holding  the  mind  open  for  further  evidence  before  settling  down  upon  a 
conviction  as  to  the  share  that  one  process  or  another  has  in  their  origin. 

Although  primarily  intended  as  a  text-book  for  use  in  schools  and  colleges, 
it  is  believed  that  a  more  advanced  class  of  readers  may  find  the  book  of  value. 
The  needs  of  the  observers  of  the  national  and  the  state  weather-services  have 
in  particular  been  borne  in  mind.  It  has  been  constantly  my  effort  to  discuss 
the  subject  in  a  rational  manner,  rather  than  to  employ  the  empirical  form 
of  statement  which  is  necessarily  adopted  in  the  official  instructions  to 
observers.  It  is  intended  to  make  careful  distinction  between  vague  sug- 
gestion and  carefully  tested  theory  ;  the  one  based  on  insufficient  observations 
and  offered  with  little  regard  to  well-determined  physical  laws  ;  the  other 
based  on  broad  generalizations,  constructed  with  careful  regard  to  the  teachings 


VI  PREFACE. 

of  physics,  and  confronted  at  every  turn  with  such  facts  as  may  serve  to 
estimate  its  value.  A  logical  combination  of  the  various  mental  processes 
called  on  in  careful  theorizing  is  regarded  as  essential  to  the  progress  of  the 
science  of  meteorology  in  contrast  with  the  simple  accumulation  of  unrea- 
soned facts.  A  precise  description  and  a  legitimate  generalization  of  the  facts 
of  observation  stand  together  on  the  inductive  side  of  the  study  ;  an  alert 
imagination  is  needed  in  deducing  combinations  of  known  physical  conditions 
by  which  the  facts  may  be  explained  ;  and  a  critical  judgment  is  called  on  in 
deciding  how  fully  the  proposed  explanations  may  be  accepted.  The  study  of 
the  movements  of  the  atmosphere,  both  general  and  local,  affords  an  excellent 
opportunity  for  the  exercise  of  these  mental  processes,  to  which  mature  readers 
are  urged  to  give  close  attention.  It  is  also  hoped  that  the  emphasis  here 
given  to  the  classification  of  the  winds  according  to  causes  and  to  the  explana- 
tion of  the  non-periodic  weather  elements  of  cyclonic  origin  may  lead  practised 
observers  to  prepare  original  descriptive  accounts  of  phenomena  under  these 
and  similar  headings,  in  which,  after  the  facts  are  carefully  determined,  the 
causes  shall  be  duly  considered.  Observant  students  of  meteorology,  well 
informed  on  the  present  condition  of  the  science,  must  find  innumerable  new 
illustrations  of  atmospheric  processes  over  the  vast  area  of  our  varied  country  : 
and  it  is  extremely  desirable  that  essays  on  these  subjects  should  be  published 
either  by  the  observer's  local  state  weather  service,  or,  better,  in  the  American 
Meteorological  Journal,  where  they  will  have  a  wider  circulation.  We  shall 
thus  come  to  learn  if  the  expected  local  convectional  clouds  occur  over  the 
sand  bars  of  the  Carolina  coast  in  quiet  summer  weather  ;  if  the  deep  valleys 
on  the  western  slope  of  the  Sierra  Nevada  in  California  furnish  nocturnal 
breezes  to  the  plain  on  which  they  open  ;  if  the  lofty  plateaus  of  Arizona 
produce  an  occasional  cold  blast  of  the  bora  type  in  winter  ;  and  if  tornadoes 
occur  distinctly  within  the  area  of  warm  southerly  winds  as  well  as  near  their 
western  margin.  We  shall  have  local  studies  of  the  variation  of  rainfall  over 
small  areas,  of  the  control  of  the  wind  by  the  topography,  of  the  amount  of 
dewfall,  and  of  the  proper  "safety  limit"  for  the  prediction  of  frost.  A 
fresh  fund  of  local  illustrations  will  thus  be  supplied,  which  will  be  found 
invaluable  by  teachers  in  supplementing  their  texts. 

W.  M.  DAVIS. 
HARVARD  COLU.'.I.. 
CAMBRIDGE,  MASS.,  August,  1893. 


TABLE  OF  CONTENTS. 


CHAPTER  I. 

PAOB 

THE  GENERAL  RELATIONS  OP  THE  ATMOSPHERE .        .        1 

SECTIONS  1  TO  12. — The  subject  of  meteorology. — The  plan  of  this  book. — 
Meteorology  as  a  branch  of  physics.  — fprigin  of  the  atmosphere  :  relation  of  the 
earth  to  other  planets.  — [Evolution  of  the  atmosphere.  — jThe  future  of  the  atmo- 
sphere. — (Composition  01  the  atmosphere.  —  Offices  of  the  atmosphere  :  relation  of 
oxygen  to  animals  and  plants.  —  Relation  of  carbonic  acid  to  plants. — Nitrogen 
and  water  vapor.  —  Dust.  —  Activities  of  the  atmosphere. 


CHAPTER   II. 

EXTENT  AND  ARRANGEMENT  OF  THE  ATMOSPHERE  ABOUT  THE  EARTH  ....        9 

SECTIONS  13  TO  20.  —  The  geosphere,  the  hydrosphere,  and  the  atmosphere.  — 
Dimensions  of  the  earth.  —  Pressure  of  the  atmosphere.  —  Barometers.  —  Down- 
ward pressure  of  the  ocean. — Isobaric  surfaces  in  the  atmosphere. — Vertical 
decrease  of  pressure  in  the  atmosphere.  —  Height  of  the  atmosphere. 

CHAPTER  III. 

THE  CONTROL  OF  ATMOSPHERIC  TEMPERATURES  BY  THE  SUN 16 

SECTIONS  21  TO  56. —  Sources  of  heat. — Nature  of  heat. — Explanation  by  hypoth- 
esis. —  Radjant  energy.  —  Radiation  from  the  sun ;  insolation.  —  Astronomical 
relations  of  sun  and  earth.  —  Distribution  of  insolation  over  the  earth.  —  Action  of 
insolation  on  the  earth.  —  Reflection.  —  Transmission.  —  Absorption.  —  Various 
effects  of  absorption. — Actinometry.  —  Radiation  from  the  earth. — Absorption 
and  radiation  by  the  atmosphere. — Vertical  temperature  gradient. — Absorption 
and  radiation  by  the  ocean. — Relation  of  diurnal  temperature  range  in  air  and 
water.  —  Absorption  and  radiation  by  the  land.  —  Inter-radiation  of  air  and  earth. 

—  Conduction. — Conduction  of  heat  between  air  and  land. — Inversions  of  tem- 
perature.—  Convection  in  water.  —  flondnrtkni  and  p.nnvpp.t.f^p  in  thp  a^pgphfrA, 

—  Mirages.  —  Dust  whirlwinds.  —  Difference  between  convection  in  liquids  and 
gases.  —  Qhange  of  temperature  in  vertical  currents.  —  Conditions  of  local  con- 
vection in  the  atmosphere.  —  Nocturnal  stability.  —  Diurnal  instability. — Explana- 
tion of  convection  by  analogy.  —  Local  convection  illustrated  by  clouds.  —  General 
vertical  distribution  of  temperature. — Review. 


Vlll  TABLE    OF    CONTENTS. 

CHAPTER   IV. 

PAGE 

THE  COLORS  OF  THE  SKY 43 

SECTIONS  57  TO  72.  — Facts  to  be  explained.  — Explanation  of  color  in  general : 
nature  of  color.  —  Selective  absorption  and  diffuse  reflection.  —  Selective  absorp- 
tion and  transmission. — Selective  scattering. — Diffraction  and  interference. — 
Refraction.  —  Explanation  of  the  colors  of  the  sky  :  the  dust  of  the  atmosphere.  — 
The  blue  of  the  sky  :  selective  scattering.  —  The  color  of  the  sun.  — Deep  blue  sky 
seen  from  mountains.  —  Sunset  and  sunrise  horizon  colors.  — The  twilight  arch.  — 
Sunset  and  sunrise  glows.  — The  red  sunsets  of  1883-84.  — Mirage  and  looming. 

CHAPTER  V. 

THE  MEASUREMENT  AND  DISTRIBUTION  OF  ATMOSPHERIC  TEMPERATURES     ...       66 

SECTIONS  73  TO  80.    Thermometry . — Thermometers. — The  sling  thermometer. 

—  Thermographs. — Maximum   and   minimum  thermometers.  —  Black  bulb  ther- 
mometer. —  Record  of  temperature.  —  Mean  temperatures.  —  Climatic  data.  — 
Isothermal  charts. 

SECTIONS  81  TO  91.  Distribution  of  Temperature  over  the  Earth.  —  Contrast 
between  the  equator  and  poles.  —  Irregularity  of  annual  isotherms.  —  General 
scheme  of  ocean  currents. —  Deflection  of  isotherms  by  ocean  currents. —  Isotherms 
for  January  and  July.  —  Poleward  temperature  gradients  in  winter.  —  Migra- 
tion of  isotherms.  —  Northern  winter  isotherms.  —  Thermal  anomalies.  —  Annual 
range  of  temperature.  —  Polar  temperatures. 

CHAPTER  VI. 
THE  PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE 77 

SECTIONS  92  TO  97.  General  Principles. — The  conditions  of  general  convec- 
tional  motion.  —  Arrangement  of  isobaric  surfaces  in  a  general  convectional 
circulation. — Conditions  of  steady  motion. —  Barometric  gradients.  —  Vertical 
components  of  a  convectional  circulation. — Application  of  general  principles  of 
convectional  motion  to  the  atmosphere. 

SECTIONS  98  TO  115.    The  Measurement  and  Distribution  of  Atmospheric  Pressure.  ' 

—  Measurement  of  atmospheric  pressure.  —  Mercurial  barometers.  —  Correction 
for  temperature.  —  Correction  for  latitude.  —  Aneroid  barometers.  —  Barographs. 

—  Diurnal  variation  of  the  barometer.  — Irregular  fluctuations  of  the  barometer.  — 
Barometer  observations.  —  Comparison  of  observations:  reduction  to  sea  level.  — 
Barometric  determination  of  altitude.  —  Barometric  charts.  —  Isobars  for  the  year. 

—  Vertical  section  of  the  atmosphere  along  a  meridian.  — Meaning  of  isobaric  lines. 

—  Interpretation  of    gradients.  —  Isobars    for  January  and    July.  —  Suggested 
explanation  for  the  distribution  of  pressure. 

nom  116  TO  123.    Observation  and  Distribution  of  the  Wind*.  —  Direction 
of  the  wind.  —  Anemoscopes.  —  Force  and  velocity  of  the  wind.  —  Anemometers. 

—  Hill  and  mountain  observatories.  —Wind  observations.  —  Reduction  of  observa- 
tions. —  The  general  winds  of  the  world. 


TABLE    OF    CONTENTS*  ix 

PAGE 

SECTIONS  124  TO  126.  Comparison  of  the  Consequences  of  the  Convectional  Theory 
with  the  Facts  of  Pressure  and  Winds.  —  General  relations  of  winds  and  pressures. 

—  Agreements  and  disagreements.  —  Low  polar  pressure  caused  by  the  prevailing 

westerly  winds. 

SECTIONS  127  TO  136.  The  Effects  of  the  Earth's  Rotation.  —  The  deflecting 
force  of  the  earth's  rotation.  —  Hadley's  theory  of  the  effect  of  the  earth's  rotation, 
1735.  —  Ferrel's  theory  of  the  effects  of  the  earth's  rotation,  1856.  —  Motion  on 
the  rotating  earth  without  friction.  —  Movement  of  the  air  on  gravitative  gradients. 

—  Deflection  of  the  winds  from  the  gradients.  —  Experimental  illustration  of  the 
deflective  effect  of  the  earth's  rotation.  —  Analogy  with  an  eddy  in  water.  —  Vorti- 
cular  circulation  of  the  atmosphere  around  the  poles.  — Cause  of  low  pressure  at 
the  poles. 

CHAPTER  VII. 

A  GENERAL  CLASSIFICATION  OF  THE  WINDS 112 

SECTIONS  137  TO  165.  —  Basis  of  classification.  —  Classification  of  the  winds 
according  to  cause.  —  Tidal  breezes.  —  Volcanic  and  accidental  winds.  —  Planetary 
winds.  —  The  members  of  the  planetary  system  of  winds.  —  Trade  winds.  —  Dol- 
drums. —  Horse  latitudes.  —  Prevailing  westerly  winds.  —  The  upper  currents.  — 
Winds  on  other  planets.  —  Terrestrial  winds.  —  Annual  migration  of  the  wind 
system.  —  Sub-equatorial  and  sub-tropical  belts.  —  Continental  winds.  —  Monsoons 
of  Asia.  —  Monsoons  of  Australia  and  elsewhere.  —  The  monsoon  influence  on  the 
terrestrial  winds.  —  Continental  obstruction  of  the  terrestrial  winds.  —  The  general 
winds.  —  Arctic  winds.  —  Diurnal  variation  in  wind  velocity  on  land.  —  Land  and 
sea  breezes.  —  The  sea  breeze  begins  off  shore.  —  Combination  of  general  and 
literal  winds.  —  Mountain  and  valley  breezes.  —  Mountain  breezes  and  inversions 
of  temperature.  —  Winds  not  yet  classified. 

CHAPTER  VIII. 
THE  MOISTURE  OF  THE  ATMOSPHERE 140 

SECTIONS  166  TO  179. — Evaporation. — Latent  heat. — Capacity.  —  Saturation. 

—  Humidity. — Absolute  and  relative  humidity. — Dewpoint. — Amount  of  evap- 
oration. —  Hygrometry.  —  Psychrometer.  —  Distribution    of    vapor  in   the  atmo- 
sphere. —  Geographic  and  periodic  variations  of  absolute  humidity.  —  Geographic 
and  periodic  variations  of  relative  humidity.  — Effect  of  water  vapor  on  the  general 
circulation  o£  the  atmosphere. 

CHAPTER  IX. 
DEW,  FROST,  AND  CLOUDS     ....:.......     154 

SECTIONS  180  TO  214.  —  Condensation. — Condensation  from  quiet  air  on  cold 
surfaces. — Cooling  retarded  by  liberation  of  latent  heat. — Dew. — Frost. — Con- 
ditions for  the  formation  of  dew  or  frost.  —  Prediction  of  frost.  —  Protection  from 
frost.  —  Valley  and  lowland  fogs.  —  Dependence  of  cloud  condensation  on  "dust." 
Siza  and  constitution  of  cloud  particles.  —  Color  of  clouds.  —  Halos  and  coronas. 

—  Cloudy  condensation   in   winds  over  cold  surfaces.  —  Clouds  formed  in  pole- 
ward winds.  —  Condensation  by  the  mechanical  cooling  of  ascending  currents.  — 


X  TABLE    OF    CONTENTS. 

PAGE 

Convectional  clouds  of  fair  summer  weather.  —  Decreased  rate  of  adiabatic 
change  of  temperature  in  cloudy  air.  —  Special  adiabatic  conditions  at  the  freezing 
point.  —  Increased  altitude  of  convectional  ascent  in  cloudy  currents.  —  Convec- 
tional clouds  over  islands  and  mountains.  —  Varied  form  of  convectional  clouds.  — 
Clouds  in  forced  ascending  currents.  —  Clouds  formed  in  atmospheric  waves.  — 
Clouds  do  not  always  float  with  the  air  currents.  —  Condensation  caused  by  con- 
duction.—  Condensation  by  upward  diffusion  of  vapor. —  Condensation  by  radiation 
from  the  atmosphere.  —  Condensation  by  mixture  of  air  currents.  —  Haze  —  Con- 
ditions that  favor  clear  sky.  — Classification  of  clouds.  —  Altitude  of  clouds.  — 
Observations  of  clouds.  —  Sunshine  records. 

CHAPTER  X. 
CTCLONIC  STORMS  AND  WINDS        ...........     183 

SECTIONS  215  TO  231.  Tropical  Cyclones.  —  Unperiodic  winds.  —  Cyclones, 
thunder  storms  and  tornadoes.  —  Tropical  cyclones.  —  Approach  and  passage  of  a 
tropical  cyclone.  —  Law  of  storms.  —  Evidence  of  convectional  action  in  tropical 
cyclones.  —  Seasons  and  regions  of  tropical  cyclones.  —  Tables  of  cyclone  fre- 
quency.—  Early  stages  of  cyclonic  action.  —  Effect  of  the  earth's  rotation. — 
Absence  of  tropical  cyclones  from  the  South  Atlantic.  —  Latent  heat  from  rain- 
fall. —  Occurrence  of  tropical  cyclones  chiefly  over  the  oceans.  —  Comparison  of 
tropical  cyclones  and  desert  whirlwinds. — The  eye  of  the  storm. — Comparison 
of  tropical  cyclones  and  the  circumpolar  whirl  of  the  planetary  circulation.  —  The 
convectional  theory  of  tropical  cyclones. 

SECTIONS  232  TO  239.  Extra-tropical  Cyclones.  —  Comparison  of  tropical  and 
extra-tropical  cyclones.  —  Unsymmetrical  form  of  extra-tropical  cyclones.  —  The 
center  of  extra-tropical  cyclones.  —  Control  of  weather  by  cyclones.  —  Non- 
convectional  origin  of  extra-tropical  cyclones.  —  Origin  of  extra-tropical  cyclones 
as  eddies  in  the  circumpolar  winds  of  the  terrestrial  circulation. — Anticyclones. 

—  Test  of  the  theories  of  extra-tropical  cyclones  and  anticyclones. 

SECTIONS  240  TO  243.  Progression  of  Cyclones.  —  Effect  of  rainfall  on  pro- 
gression. —  Effect  of  progression  on  the  velocity  and  direction  of  cyclonic  winds. 

—  Veering  and  backing  of  the  winds  caused  by  the  passage  of  cyclones. 

SECTIONS  244  TO  250.  Cyclonic  and  Anticyclonic  Winds. — Frequence  of 
cyclonic  winds  in  the  temperate  zone.  —  The  warm  wave  or  sirocco.  —  The 
cold  wave.  —  The  bora.  —  The  foehn  or  chinook.  —  The  anticyclonic  calm.  — 
Comparison  of  the  foregoing  examples. 

CHAPTER  XI. 
LOCAL  STORMS 248 

SECTIONS  251  TO  265.    Thunderstorms. — Thunder  storms  and  thunder  squalls. 

—  The  passage  of  a  thunder  storm.  —  Observation  of  thunderstorms.  —  Convec- 
tional action  in  thunder  storms.  —  Geographical  distribution  of  thunder  storms.  — 
Mountain  thunderstorms.  —  Relation  of  thunderstorms  to  cyclones. — The  pro- 
gression of  thunder  storms.  — The  thunder  squall.  —  Nocturnal  thunder  storms.  — 
Atmospheric  electricity.  —  Lightning.  —  Thunder.  —  Lightning  strokes  and  light- 
ning rods.  —  Aurora  boreal  is. 


TABLE    OF   CONTEXTS.  XI 

PAOK 

SECTIONS  266  TO  276.  Tornadoes  and  Waterspouts.  — Tornadoes. — Regions  and 
seasons  of  occurrence.  —  Convectional  origin  of  tornadoes.  —  The  vortex  of  tor- 
nadoes. —  Fen-el's  theory  of  tornadoes.  —  Central  low  pressure  of  tornadoes.  — 
The  tornado  funnel  cloud.  —  Progression  of  tornadoes.  —  Protection  from  tor- 
nadoes. —  Observation  of  tornadoes.  —  Waterspouts. 


CHAPTER   XII. 

THE  CAUSES  AND  DISTRIBUTION  OF  RAINFALL       ........     286 

SECTIONS  277  TO  290.  Causes  of  Rainfall.  —  Snow.  —  Hail.  —  Conditions  of 
rainfall.  —  Cooling  caused  by  rainfall.  —  Variation  of  rainfall  with  altitude.  — 
Measurement  of  rainfall. — Records  of  rainfall. — Relation  of  rainfall  and  agri- 
culture. —  Snowfall.  —  Ice  storms.  —  Relation  of  rainfall  and  forests.  —  Artificial 
rain.  —  Rainbows. 

SECTIONS  291  TO  303.  Correlation  of  Eainfall  with  the  Circulation  of  the 
Atmosphere.  —  Equatorial  rains.  —  Trade  wind  rains.  —  Trade  wind  deserts.  — 
The  horse  latitudes.  —  The  stormy  rainfall  of  the  westerly  winds.  —  Arid  regions 
of  the  westerly  winds.  —  Contrasts  of  torrid  and  temperate  rainfalls.  —  Rainfall 
of  high  latitudes.  —  Migration  of  rain  belts.  —  Sub-equatorial  rains.  —  Sub-tropical 
rainfall.  —  Effects  of  clouds  and  rainfall  on  the  general  circulation  of  the 
atmosphere. 

CHAPTER   XIII. 
WEATHER .310 

SECTIONS  304  TO  316.  Weather.  —  Weather  elements.  —  Controls  of  weather 
changes.  —  Weather  of  the  torrid  zone.  —  The  trade  wind  belts.  —  The  equatorial 
belt.  —  The  sea  breeze.  —  Weather  of  the  temperate  zones.  —  The  south  temperate 
zone. —  The  north  temperate  zone. —  Summer  weather  in  the  central  United  States. 

—  Winter  weather  in  the  central  United  States.  — Weather  of  the  frigid  zones. 

SECTIONS  317  TO  327.  Weather  Observation  and  Prediction.  —  Weather  obser- 
vations. —  Weather  Bureau  of  the  United  States.  —  Weather  maps.  —  Methods  of 
weather  prediction.  —  Distribution  of  predictions.  —  Weather  and  storm  signals. 

—  State  weather  services. — Private  meteorological  observatories. —  Foreign  weather 
services.  —  Weather  proverbs  and  weather  lore.  —  Weather  cycles. 


CHAPTER   XIV. 
CLIMATE 333 

SECTIONS  328  TO  343.  —  Climate.  —  Climatic  zones  and  subdivisions.  —  The 
torrid  zone.  —  The  oceans  of  the  torrid  zone.  —  The  lands  of  the  torrid  zone.  — 
The  transitional  subtropical  areas.  —  The  south  temperate  zone.  —  The  north 
temperate  zone.  —  The  coasts  of  the  north  temperate  zone.  —  The  mountain 
climate  of  the  temperate  zone.  —  The  frigid  zones.  —  Climatic  control  of  habit- 
ability.  —  Local  control  of  climate.  —  Periodic  variations  of  climate.  —  Secular 
rariations  of  climate.  —  Geological  changes  of  climate. 


ACKNOWLEDGMENT  OF  FIGURES  AND  CHARTS. 


(Figures  not  named  in  this  list  are  original.) 

3a.    Ground  temperature.     Bavarian  Meteorological  Observations. 

7,  13,  23,  28,  46,  47.  Meteorological  instruments.  Henry  J.  Green,  instrument 
maker,  New  York. 

9,  24.  Richard  thermograph  and  barograph.  Glaenzer,  agent,  80  Chambers  street, 
New  York. 

14,  15,  36,  37,  42,  43.  Isotherms  and  isobars  of  Spain.  Adapted  from  Teisserenc 
de  Bort,  Annales,  Bureau  Centr.  MeteoroL  tie  France,  1879. 

16,  17.    Isanomalous  temperatures.     Batchelder,  Amer.  MeteoroL  Journal,  Vol.  X. 

18.    Equal  annual  temperature  ranges.     Connolly,  Amer.  MeteoroL  Journal,  Vol.  X. 

29,  30.  Winds  at  Kinderhook  and  Utica,  N.  Y.  Hough,  Climate  of  the  State  of  New 
York,  1857. 

38,  39.  Winds  of  the  Indian  ocean.  Koppen,  in  the  Atlas;  Indischer  Ozean, 
Deutsche  Seewarte,  Hamburg,  Germany. 

40,  41.   Winds  of  the  Atlantic  ocean.    Ditto,  Segelhandbuch  fiir  den  Atlantischen  Ozetm. 

44.    Winds  at  Chicago.     Hazen,  U.  S.  Signal  Service,  Note  VI. 

53,  54,  55,  58,  63.  North  Atlantic  storms.  Hayden,  Pilot  Charts  of  the  Nortlt 
Atlantic,  U.  S.  Hydrographic  Office. 

57.    Tracks  of  West  Indian  hurricanes.     Redfield,  Amer.  Jour.  Science,  1854. 

59.  Doldrums  of  the  Atlantic.  Prepared  from  Pilot  Charts  of  the  North  Atlantic 
and  other  sources. 

61.  North    Atlantic    storm.      Synchronous    Weather    Charts    of  the    North   Atlantic, 
Meteorological  Council,  London,  1886. 

62.  Circumpolar  storm  tracks.     Loomis,  Contributions  to  Meteorology,  1885. 

64,  67,  106.    Reduced  and  adapted  from  daily  weather  maps,  U.  S.  Signal  Service. 
•  '••"),  66,  68,  69.    Winds  and  clouds  in  cyclones  and  anticyclones.      Clayton,  Amer. 
MeteoroL  Journal,  August,  1893. 

74  to  79.    Cold  wave  of  January,  1886.     Davis,  Science,  1886. 

81.    Mediterranean  cyclone.     Hann,  in  Berghaus'  Phyniache  Atlas,  sheet  :Ji;. 

90.  Thunder  squall  in  Iowa.     Hinrichs,  Iowa  Weather  Report,  1877. 

91.  Thunder  squall  in  Tennessee.    Clayton,  Amer.  MeteoroL  Journal,  November,  1884. 

92.  Thunderstorm  in  New  England.     Davis,  Proc.  Amer.  Anul.,  Boston,  1886. 

93.  Thunderstorm  in  Germany.     Koppen,  Ann.  tier  JI//</r<i(/r<t/>/iie,  1882. 
98.    Cyclone  and  thunder  storms.     Hazen,  U.  S.  Signal  Service,  Note  XX. 

101.  -Distribution   of   tornadoes   in    cyclones.      Davis    and    Curry,  Amer.   MeteoroL 
Journal,  January,  1890. 

105.    Salinity  of  the  Atlantic  ocean.     Buchanan.  HI /torts  of  the  Challenger  Expedition. 

(harts  I  to  VI.      Isotherms  and  I.sobars.      Buchan,  Report  on  Atmotpheric  Circulation^ 
Challenger  Expedition,  London,  1889. 


ELEMENTARY    METEOROLOGY. 


CHAPTER   I. 

THE  GENERAL  RELATIONS  OP  THE  ATMOSPHERE. 

1.  The  subject  of  meteorology.     We  dwell  on  the  surface  of  the  land ; 
we  sail  across  the  surface  of  the  sea  ;  but  we  live  at  the  bottom  of  the  atmos- 
phere.    Its  changes  pass  over  our  heads  ;    its  continual  fluctuations  control 
our  labors.     Whether  our  occupation  is  indoor  or  out,  on  land  or  at  sea,  we 
are  all  more  or  less  influenced  by  changes  from  the  clear  sunshine  of  blue 
skies  to  the  dark  shadows  under  clouds ;  from  the  dusty  weather  of  droughts 
to  the  rains  of  passing  storms  ;  from  the  enervating  southerly  winds  to  the 
bracing  currents  from  the  north.      Few  persons  fail  to  raise  some  question 
now  and  then  concerning  the  causes  and  processes  of  these  changes  ;  some 
inquire  more  earnestly,  desiring  to  inform  themselves  carefully  on  the  subject. 
Xo  school  study  suggests  more  frequent  questions  from  scholars,  or  allows 
more  educative  replies  from  teachers  than  meteorology,  the  science  of  the 
atmosphere. 

It  is  the  author's  intention  in  preparing  this  book  to  place  before  both 
readers  and  students  an  outline  of  what  is  now  known  in  the  domain  of 
meteorology  ;  to  give  a  condensed  account  of  the  present  condition  of  the 
science,  without  too  much  technical  language  or  argumentative  demonstration. 
All  available  sources  of  information  have  been  drawn  upon  in  the  effort  to 
make  the  various  chapters  represent  the  position  of  the  modern  meteorologist. 

2.  The  plan  of  this  book  may  be  concisely  stated.     The  origin  and  uses  of 
the  atmosphere  are  first  considered,  with  its  extent  and  arrangement  around 
the  earth.     Then,  as  the  winds  depend  on  differences  of  temperature  over  the 
world,  the  control  of  the  temperature  of  the  atmosphere  by  the  sun  is  dis- 
cussed, and  the  actual  distribution  and  variations  of  temperature  are  examined. 
Xext  follows  an  account  of  the  motions  of  the  atmosphere  in  the  general  and 
local  winds  ;    in   the   steady   trades   of  the   torrid  zone   and  in   the  variable 
westerly  winds  of   our  latitudes.      The  moisture  of  the  atmosphere  is  then 
studied  with  regard  to  its  origin,  its  distribution  and  its  condensation  into 
dew,  frost  and  clouds.     After  this,  we  are  led  to  the  discussion  of  those  more 
or  less  frequent  disturbances  which  we  place  together  under  the  name  of 
storms  ;  some  of  them  being  large,  like  the  great  cyclones  or  areas  of  low 
pressure  on  our  weather  maps ;  some  of  them  very  small,  like  the  destructive 
tornadoes.     The  effect  of  these  storms  and  of  other  processes  in  the  precipita- 


A  1 :  V    M  ETEOROLOGY. 

tion  of  moisture  as  rain,  snow  and  hail  is  next  considered.  Closing  chapters 
are  then  given  to  the  succession  of  atmospheric  phenomena  that  ordinarily 
follow  one  another,  on  which  our  local  variations  of  weather  depend,  together 
with  some  account  of  weather  prediction  ;  and  another  on  the  recurrent 
average  conditions  that  we  may  expect  -in  successive  seasons,  repeated  year 
after  year,  which  we  call  climate. 

3,  Meteorology  as  a  branch  of  physics.    All  the  conditions  and  phenomena 
of  the  atmosphere  are  illustrations  of  the  principles  of  physics.     The  proper- 
ties of  gases  and  vapors,  and  the  laws  of  heat  and  motion  are  here  exemplified 
on  a  great  scale,  vastly  larger  than  that  usually  considered  in  laboratory  ex- 
periments ;  but  the  difference  of  scale  does  not  in  any  way  affect  the  applica- 
tion of  physical  laws. 

It  is  therefore  essential  that  the  student  should  have  at  least  a  fair 
elementary  knowledge  of  physics,  gained  if  possible  from  laboratory  experi- 
ments as  well  as  from  the  study  of  text-books,  before  entering  on  the  subject 
of  meteorology.  If  any  such  terms  as  the  following  are  not  precisely  under- 
stood, they  should  be  carefully  studied  again  in  a  good  book  on  physics  as 
they  are  encountered  in  these  pages  :  mass,  volume,  density  ;  inertia,  force, 
velocity,  rotation,  centrifugal  force  ;  gravitation,  gravity,  weight ;  atom, 
molecule ;  solid,  liquid,  gas ;  expansion,  heat,  temperature,  specific  heat, 
latent  heat. 

ORIGIN   OF   THE   ATMOSPHERE. 

4.  Relation  of  the  earth  to  the  other  planets.     The  atmosphere,  chiefly  a 
mixture  of  nitrogen  and  oxygen,  is  thought  to  be  a  thin  remainder  of  a  once 
much  larger  volume  of  denser  gases  and  vapors.     Our  understanding  of  this 
comes  best  by  looking  into  the  early  history  of  the  earth  and  the  other  planets 
that  accompany  the  sun.      All  these  planets    are    nearly  spherical   bodies, 
rotating  as  far  as  known  from  west  to  east  and  moving  around  their  orbits  in 
the  same  direction.     Most  of  them  are  accompanied  by  one  or  more  satellites, 
revolving  again  in  the  same  direction.     The  sun  turns  on  its  axis  in  the  same 
way  as  the  planets  revolve  around  it. 

The  resemblances  among  these  bodies1  are  indeed  so  numerous  and  so  strik- 
ing that  it  has  come  to  be  generally  believed  that  the  matter  of  which  they 
are  composed  was  once  scattered  thinly  through  an  enormous  space,  making  a, 
vast  cloud  or  nebula,  similar  to  various  nebulae  that  may  still  he  seen  by  tin- 
telescope  in  remote  parts  of  the  sky  ;  that  the  gradual  falling  together  of 
most  of  the  cloudy  mass  about  its  center  produced  the  sun,  while  the  planets 
were  formed  by  the  gathering  together  of  much  smaller  amounts  of  matter 
about  subordinate  centers.  The  correspondence  of  rotary  motions  now  ob- 
servable is  regarded  as  a  common  inheritance  from  the  slow  turning  of  the 


THE  GENERAL  RELATIONS  OF  THE  ATMOSPHERE.  8 

original  nebula  from  which  the  solar  system  is  supposed  to  have  been  evolved ; 
and  this  theory  of  the  origin  of  the  sun  and  the  planets  is  consequently  called 
the  nebulur  hypothesis.  When  the  scattered  parts  of  the  early  nebula  were 
gathered  together,  the  larger  bodies  that  they  formed  are  believed  to  have 
possessed  an  excessively  high  temperature.  The  sun,  being  the  largest  of  all, 
still  retains  much  of  its  primitive  heat.  The  earth,  being  smaller,  has  now 
cooled  to  a  low  temperature  on  its  surface  ;  a  large  amount  of  heat  is,  however, 
still,  retained  within  the  earth. 

•'."-'  •'-'.•.  ..--.•.-------•••  '''/'; 

5.  Involution  of  the  atmosphere.  In  the  early  youth  of  the  earth,  when 
according  to  the  hypothesis  its  surface  temperatures  were  high,  many  sub- 
stances that  might  later  be  condensed  at  lower  temperatures  in  the  liquid 
ocean  or  the  solid  crust,  would  then  exist  in  the  atmosphere.  Such  an  atmo- 
sphere would  be  dense  and  vaporous  ;  heavy  clouds  would  hang  in  its  upper 
layers,  and  drenching  rains  would  fall  towards  the  glowing  earth,  only  to  be 
boiled  off  again  as  they  approached  it ;  until  at  last  by  a  long  process  of 
slow  cooling  through  untold  ages,  more  and  more  condensation  would  take 
place,  reducing  the  volume  of  the  atmosphere  to  moderate  measures,  when  only 
a  small  share  of  its  original  mass  would  remain.  Changes  of  this  kind  would 
take  place  faster  on  the  smaller  planets,  slower  on  the  larger  ones  ;  and  this 
seems  to  be  the  fact  in  our  own  system.  The  moon,  a  comparatively  small 
body,  appears  to  have  lost  all  its  atmosphere.  Jupiter,  much  larger  than  the 
earth,  appears  still  to  possess  a  very  cloudy  atmosphere  ;  and  from  the  great 
brightness  of  this  planet,  astronomers  have  been  led  to  suppose  that  its  body 
is  still  so  hot  as  to  be  somewhat  luminous.  The  sun,  vastly  larger  than  any 
of  the  planets,  still  retains  an  atmosphere  of  great  volume  at  excessively  high 
temperatures,  which  its  small  neighbors  have  long  ago  lost. 

Our  earth  occupies  an  intermediate  position.  Some  of  the  more  volatile 
mineral  substances  in  the  rock-crust  of  the  earth  presumably  at  an  early  time 
made  a  part  of  the  atmosphere,  but  all  these  have  long  ago  left  it.  Nearly  all 
of  the  water  that  must  have  once  been  boiled  off  in  the  steamy  atmosphere  of 
early  times  has  now  condensed  upon  the  cooled  surface  of  the  earth,  forming 
the  deep  oceans.  Some  of  the  gases  themselves,  particularly  the  oxygen  of  the 
air,  must  have  been  much  diminished  by  combining  with  the  surface  rocks  of 
the  earth's  crust  and  rusting  them. 

It  is  also  possible  that  the  early  atmosphere  has  been  diminished  not  only  by 
condensation  and  combination  on  the  earth,  but  also  by  flying  away  from  the 
earth.  If  lighter  and  more  active  gases,  such  as  hydrogen,  ever  existed  free  in 
the  atmosphere,  it  may  be  plausibly  supposed  that  they  have  escaped  from 
the  earth's  attraction  and  passed  out  to  open  space,  to  be  gradually  gathered 
around  larger  planets  or  suns.  The  absence  of  even  the  heavier  gases  of  our 
atmosphere  around  smaller  bodies,  such  as  the  moon,  has  been  thus  accounted 


4  ELEMENTARY    METEOROLOGY. 

for.  The  atmosphere,  at  the  bottom  of  which  we  live,  must,  therefore,  be 
regarded  simply  as  the  thin  residual  of  the  much  vaster  early  atmosphere  that 
once  surrounded  the  earth. 

6.  The  future  of  the  atmosphere.     We  may  not  only  look  back  into  the 
past ;  we  may  peer  forward  into  the  future,  and  speculate  as  to  the  further 
changes  still  in  store  for  the  atmosphere.     The  earth  already  having  cooled 
greatly  by  the  comparatively  rapid  loss  of  its  own  heat,  the  further  lowering 
of  temperature  on  its  surface  depends  chiefly  on  the  slower  cooling  of  the  sun. 
When  the  sun  at  last  becomes  cold  and  dark,  all  the  water  vapor  will  have 

.  forsaken  the  atmosphere,  and  our  oceans  will  have  frozen  solid.     The  air  will 

be  absolutely  calm,  and  all  the  dust  will  settle  from  it,  leaving  it  a  pure,  clean 

gas.     More  of  the  oxygen  will  have  then   combined  with  the  rocks  of  the 

earth's  crust ;  possibly  nearly  all  of  it  may  by  that  time  have  been  withdrawn 

from  the  atmosphere  ;  but  the  nitrogen,  the  inert  element   of   the   air,  will 

remain,  little  changed  from  its  present  amount.     We  cannot  easily  imagine  any 

process  by  which  the  nitrogen  of  the  atmosphere  will  be  disposed  of,  unless 

j  the  surface  of  the  earth  becomes  so  absolutely  cold  that  the  gaseous  condition 

HWiould  be  lost  and  the  nitrogen  should  condense  as  a  solid  on  the  frozen  earth. 

The  future  does  not,  according  to  these  speculations,  appear  to  have  in 

store  so  great  a  change  as  has  occurred  in  the  past.     When  the  earth  is  cold 

and  the  sun  dark,  the  atmosphere  will  be  somewhat  thinner  than  now,  but  its 

decrease  in  volume  will  not  be  nearly  so  great  in  the  future,  while  the  sun 

cools,  as  it  has  been  in  the  past,  during  the  cooling  of  the  earth. 

The  changes  in  the  condition  of  the  atmosphere,  here  so  briefly  reviewed, 
have  required  the  passage  of  untold  ages  of  time.  All  the  millions  of  years 
during  which  the  earth  has  already  possessed  temperatures  fitted  for  the 
existence  of  life  on  its  surface,  form  but  a  short  middle  chapter  between  the 
much  greater  duration  of  its  ardent  youth,  long  past,  and  its  cold  old  age,  yet 
to  come.  While  we  may  gain  some  general  conception  of  the  changes  that 
have  taken  place  and  that  are  yet  in  store  for  the  earth,  the  time  measured  by 
these  changes  passes  our  comprehension. 

7.  Composition  of  the  atmosphere.     As  at  present  constituted,  pure,  dry 
air,  from  which  the  dust,  water  vapor,  and  carbonic  acid  have  been  taken 
away,  consists  of  oxygen  and  nitrogen  in  the  proportion  of  21  to  79  parts  by 
volume.    These  two  gases  are  not  chemically  combined,  but  are  simply  mixed 
together.     Their  mixture   is   very  perfect,  and  extraordinarily  uniform   the 
world  over.      Analyses  of   samples  of  air  collected  from  all  the  continents, 
from  many  parts  of  the  oceans,  from  sea-level,  from  mountain  tops,  and  from 
lofty  balloon  voyages  show  hardly  any  variation  in  the  proportion  of  these 
two  chief  constituents.     This  is  because  tin-  atmosphere  is  extremely  mobile, 


THE  GENERAL  DELATIONS  OF  THE  ATMOSPHERE.          5 

and  because  gases  possess  the  property  of  spontaneous  mixture  or  diffusion, 
whereby  inequality  of  composition  is  soon  lost. 

The  ordinary  atmosphere  possesses  in  addition  to  the  oxygen  and  nitrogen 
a  small  part,  about  three-hundredths  of  one  per  cent.,  of  carbonic  acid.  This 
varies  slightly,  being  a  trifle  less  by  day  and  in  the  summer,  than  by  night 
and  in  the  winter  ;  but  the  changes  of  its  proportion  are  extremely  minute. 
There  is  also  a  variable  quantity  of  water  vapor,  sometimes  locally  amounting 
to  three  per  cent,  of  the  air  by  weight,  but  generally  much  less.  Besides  these, 
there  are  occasionally  minute  quantities  of  accidental  constituents,  produced  by 
lightning,  such  as  ammonia,  nitrous  acid,  and  ozone,1  in  addition  to  various 
microscopic  solid  particles,  such  as  dust  from  the  land,  salt  from  the  sea,  the 
pollen  and  spores  of  plants,  and  innumerable  organic  germs. 

Nitrogen,  which  constitutes  the  largest  part  of  the  atmosphere,  is  a  com- 
paratively rare  element  in  the  earth.  The  probable  explanation  of  its  large 
amount  in  the  atmosphere  is  found  in  its  chemical  inertness.  It  does  not 
easily  combine  with  other  substances,  and  henjpe,  although  a  rare  element  in 
the  earth  as  a  whole,  is  common  in  the  atmosphere  from  having  been  left  over 
at  the  time  when  other  elements  united  to  form  liquid  or  solid  substances. 

Oxygen,  on  the  other  hand,  is  one  of  the  commonest  substances  in 
earth.  It  forms  a  large  proportion  of  the  waters  of  the  ocean  and  of  the 
superficial  rocks  of  the  earth's  crust.  It  constitutes  a  small  share  of  the 
atmosphere,  not  because  it  was  in  small  quantity  in  the  begining,  but  pre- 
sumably because  its  original  abundance  was  actively  reduced  by  uniting  with 
other  substances  in  chemical  compounds.  In  spite  of  its  having  been  orig- 
inally in  great  quantity,  it  now  makes  the  smaller  part  of  the  atmosphere. 

Carbonic  acid,  a  compound  of  carbon  and  oxygen,  is  trifling  in  amount  in 
the  atmosphere  and  yet  is  of  essential  importance  to  the  vital  processes  of 
plants,  as  will  be  seen  in  the  next  sections.  It  is  given  off  with  water  vapor 
and  other  gases  in  volcanic  eruptions  ;  its  carbon  is  taken  in  by  plants,  which 
in  times  long  past  have  thus  stored  up  great  quantities  of  carbon  in  coal  beds. 
Hence  the  proportion  of  carbonic  acid  has  probably  varied  during  the  evolu- 
tion of  present  conditions' ;  though  it  should  not  be  inferred  that  all  of  the 
carbon  now  existing  as  coal  was  at  any  one  time  combined  with  oxygen,  and 
thus  added  to  the  store  of  carbonic  acid  in  the  atmosphere. 

In  some  volcanic  districts,  carbonic  acid  is  given  off  plentifully  enough  to 
accumulate  in  the  hollows  and  render  the  air  poisonous.  An  example  of  this 

1  Ozone  is  an  allotropic  form  of  oxygen ;  its  molecule  consisting  of  three  oxygen  atoms, 
while  the  ordinary  oxygen  molecule  consists  of  two  atoms.  Ozone  has  a  peculiar  odor, 
whence  its  name.  It  acts  as  an  oxydizing  agent,  because  it  easily  gives  up  one  of  its  atoms, 
thus  returning  to  the  condition  of  ordinary  oxygen.  Its  presence  is  generally  tested  by 
means  of  this  property  ;  the  rate  of  change  of  some  easily  oxidized  substance  being  taken  to 
measure  the  amount  of  ozone  present  at  the  time.  This  test,  however,  is  not  accurate,  as  the 
same  change  may  be  caused  by  other  atmospheric  impurities. 


6  ELEMENTARY    METEOROLOGY. 

is  found  in  Death  Gulch,  a  ravine  in  our  Yellowstone  National  Park,  where 
the  proportion  of  the  gas  emitted  from  the  ground  is  sufficient  to  suffocate 
animals  that  stray  there. 

8.  Offices  of  the  atmosphere :  relation  of  oxygen  to  ani~  Js  and  plants. 
The  peculiar  relation  of  the  atmosphere  to  organic  life       ,y  be  explained  by 
analogy  with  the  case  of  an  ordinary  steam  engine.     A  steam  engine  burns 
fuel,  such  as  coal  or  wood,  in  order  to  gain  energy  to  do  the  work  that  it  has  to 
peril  trin.     All  plants  and  animals  burn  fuel,  that  is,  some  part  of  their  organic 
substance,  for  the  same  purpose.     Engines  do  their  work  by  the  energy  of 
high-pressure  steam  that  has  been  formed  from  water  and  raised  to  a  high 
temperature  by  the  heat  from  the  fiery  combustion  of  the  fuel  in  a  grate  or 
fire-box  close  to  the  boiler  ;  and  the  essential  supporter  of  this  fiery  combus- 
tion is  the  oxygen  of  the  air.     The  work  performed  by  animals,  such  as  walk- 
ing, swimming,  flying  and  everything  else  in  which  resistance  is  overcome,  is 
done  by  means  of  the  energy  gained  from  a  fireless  combustion,  a  slow  com- 
bination of   some  of  their  blood  with  the   oxygen  of   the  atmosphere  j   the 
oxygen  that  fish  find  dissolved  in  the  water  having  been  gained  from  the 
atmosphere  above. 

All  plants  also  have  some  work  to  do  ;  truly  a  trifle  compared  to  that 
accomplished  by  animals,  but  still  properly  named  work  ;  either  in  the  lifting 
of  the  sap  from  the  roots  to  the  leaves,  or  in  the  moving  of  the  roots,  the 
tendrils,  or  the  leaves  ;  and  like  animals,  they  gain  the  energy  for  this  work 
by  a  slow  combustion,  a  combination  of  part  of  their  organic  substance  with 
the  oxygen  of  the  air,  which  goes  on  in  all  their  living  cells. 

In  both  animals  and  plants,  the  combustion  here  referred  to  is  associated 
with  the  process  of  respiration,  corresponding  to  the  draft  of  the  fire  in  an 
engine.  Kespiration  includes  the  inhalation  of  a  certain  amount  of  air,  the 
combination  of  part  of  the  oxygen  in  the  inhaled  air  with  some  of  the  organic 
substance  of  the  plant  or  animal,  and  the  exhalation  of  the  products  of  com- 
bustion with  the  unused  air.  The  process  is  much  alike  in  plants  and  animals, 
differing  rather  in  quantity  than  in  kind,  although  performed  by  very  different 
organs.  The  products  of  .exhalation  are  chiefly  carbonic  acid  and  water.  In 
the  case  of  animals,  these  are  accompanied  by  certain  noxious  organic  vapors, 
and  it  is  from  the  latter  that  the  unpleasant  odor  and  oppressive  feeling  arise 
in  poorly  ventilated  rooms. 

9,  Relation   of   carbonic   acid  to  plants.     In  another  way,  plants  and 
animals  are  strongly  unlike.     This  is  in  respect  to  the  food  on  which  they  live. 
Animals  require  some  organic  substance  for  food,  either  from  plants  or  from 
other  animals.     The   food  is  used  to  build  up  their  bodies,  to  repair  their 
waste,  or  to  support  the  slpw  combustion  that  has  already  been  referred  to,  by 


THE  GENERAL  RELATIONS  OF  THE  ATMOSPHERE.  7 

means  of  which  they  can  do  work.  The  higher  plants,  on  the  other  hand,  live 
as  a  rule  on  inorganic  substances,  which  they  derive  from  two  sources.  The 
sap,  consisting  mostly  of  water  with  a  small  amount  of  mineral  and  organic 
substance  dissolved  in  it,  comes  from  the  earth  through  the  roots.  It  rises 
through  the  stem  and  branches  to  the  leaves,  where  much  of  it  evaporates. 
But  another  part  of  the  food  of  the  higher  plants  comes  from  the  carbonic 
acid  of  the  air,  and  it  is  in  this  relation  to  the  atmosphere  that  plants  and 
animals  are  so  unlike.  The  carbonic  acid  of  the  atmosphere  is  of  no  use  to 
animals.  It  is  given  out  in  the  exhalation  of  their  breath,  just  as  it  is  ex- 
haled, truly  in  very  small  quantity,  from  the  breathing  cells  of  plants  ;  but  in 
plants  the  carbonic  acid  of  the  atmosphere  is  taken  in  by  the  green  cells  of 
the  leaves,  and  under  the  action  of  sunshine  it  is  decomposed,  the  carbon  being 
retained  and  the  oxygen  given  out.  This  process,  therefore,  goes  on  only  in 
the  daytime,  and  not  both  day  and  night,  as  in  the  case  of  breathing.  The 
sap  and  the  carbon  gained  from  the  earth  and  the  air,  constitute  the  food  of 
the  higher  plants.  From  these,  the  plant  builds  up  its  tissues,  repairs  its 
waste,  and  supplies  the  fuel  for  the  very  gentle  combustion  that  goes  on  in  its 
breathing  cells.  The  oxygen  liberated  by  the  decomposition  of  the  carbonic 
acid  in  the  green  cells  goes  off  to  the  air,  and  thus  about  balances  the  con- 
sumption of  oxygen  by  plants  and  animals.  It  therefore  appears  that  the 
relations  of  plants  and  animals  to  the  oxygen  and  carbonic  acid  of  the  atmo- 
sphere are  in  part  alike  and  in  part  very  unlike. 

10.  The  nitrogen  of  the  atmosphere  has  already  been  referred  to  "as  a  very 
inert  element.  It  does  not  appear  to  have  any  direct  use.  By  increasing  the 
density  of  the  atmosphere,  it  enables  the  voice  to  be  heard  further  than  it 
would  be  in  a  thinner  gas  ;  it  makes  flying  easier  for  birds  and  insects  ;  it 
makes  the  wind  stronger  and  more  serviceable  in  turning  windmills  and  blow- 
ing the  sails  of  ships  ;  by  diluting  the  oxygen,  it  diminishes  the  activity  of 
combustion,  which  in  an  atmosphere  of  pure  oxygen  would  be  excessive  ;  but 
it  does  not  appear  to  act  in  any  direct  way. 

Water  vapor,  the  most  variable  component  of  the  atmosphere,  is  of  extreme 
importance  in  many  regards.  The  movement  of  vapor  in  the  atmosphere 
constitutes  one  member  in  the  continuous  circulation  of  the  waters  of  the 
world,  beginning  in  the  evaporation  of  water  from  the  ocean  surface,  passing 
then  as  vapor,  carried  by  the  winds,  until,  condensing  in  clouds  and  falling  as 
rain  or  snow,  it  reaches  the  land  or  the  sea  ;  that  part  which  falls  upon  the 
land  gathers  in  streams  and  rivers  running  down  the  slopes  of  the  surface  and 
bearing  the  waste  of  the  land  with  it  to  the  sea. 

• 

•11.  Dust.  The  solid  impurities  of  the  atmosphere  are  of  varied  nature. 
Besides  organic  particles  of  many  kinds,  mineral  dust  is  raised  into  the  air  by 


ELEMENTARY    METEOROLOGY. 

the  winds  ;  the  coarser  particles  soon  settle  down  again,  but  the  finer  ones 
may  remain  in  suspension  for  months  or  years.  The  spray  blown  by  the 
winds  from  ocean  waves  may  evaporate,  leaving  its  finely  divided  salt  in  the 
air,  when  it  may  be  carried  many  miles  before  settling  or  being  washed  down 
in  rain.  Explosive  volcanic  eruptions  also  furnish  large  quantities  of  dust 
particles,  which  may  be  carried  by  the  outbursting  gases  high  into  the  upper 
atmosphere  (section  71).  The  lower  air,  especially  in  dusty  regions,  contains 
thousands  or  even  hundreds  of  thousands  of  particles  of  one  kind  or  another 
in  a  cubic  inch.  Over  the  oceans  or  high  in  the  atmosphere,  the  air  is  rela- 
tively clean  and  pure.  We  shall  see  reason  further  on  for  believing  that 
the  dust  thus  suspended  in  the  atmosphere  plays  an  important  part  in  deter- 
mining its  temperature,  as  well  as  in  illuminating  the  sky  under  sunshine  and 
determining  its  color ;  and  if  certain  physical  experiments  in  the  laboratory 
apply  also  in  the  greater  scale  in  nature,  the  dust  suspended  in  the  air  may  be 
of  much  use,  if  not  essential,  in  determining  the  production  of  fogs  and  clouds, 
and  thus  of  rain. 

12.  Activities  of  the  atmosphere.  The  air  as  a  whole  may  be  regarded 
first  as  a  medium  in  which  a  great  variety  of  the  minutest  forms  of  organic 
life  are  carried  about.  The  germs  upon  which  the  decomposition  of  organic 
matter  depends,  and  which  determine  in  so  many  cases  the  occurrence  of 
disease,  are  carried  easily  by  the  lightest  movement  of  the  air.  The  spores 
and  pollen  of  plants  are  widely  distributed  from  their  source  ;  the  winged 
seeds  of  many  plants  are  carried  for  less  distances.  Most  birds  and  many 
insects  use  the  air  to  support  their  flight,  and  occasionally  mammals,  reptiles, 
and  even  fish  do  the  same.  Air  in  motion  serves  to  drive  sailing  vessels  over 
the  sea,  to  turn  t'ue  wheels  of  wind-mills,  and  to  support  balloons.  The  latter 
use,  although  at  present  rare  and  comparatively  dangerous,  is  probably  des- 
tined to  become  of  much  greater  service  in  the  future.  As  a  geological  agent, 
the  moving  air  is  of  great  importance  in  transporting  fine  dust  from  place  to 
place,  as  well  as  in  carrying  the  vapor  by  which  rivers  are  fed,  as  has  already 
been  mentioned.  Moreover,  the  winds  blowing  over  the  sea  raise  waves,  which 
beat  upon  the  shoals  and  the  shores,  and  grind  the  land  down  beneath  the 
level  of  the  waters.  The  winds  drive  the  waves  along,  and  thus  create  cur- 
rentfl  which  not  only  largely  determine  the  distribution  of  the  forms  of  life  in 
the  sea,  but  also  have  an  extraordinary  effect  in  the  distribution  of  tempera- 
ture in  the  air.  Finally,  it  is  the  oxygen  of  the  air  which  supports  not  only 
the  combustion  that  is  so  essential  in  all  plants  and  animals,  but  also  the 
more  active  combustion  of  all  engines  which  are  driven  by  fires,  and  the  com- 
bustion which  directly  or  indirectly  serves  to  give  light  at  night.  In  these 
many  ways,  our  own  life  arid  activities,  the  life  of  all  plants  and  animals,  the 
life  of  the  earth  itself,  all  depend  upon  the  atmosphere  which  lies  around  us. 


ARRANGEMENT  OF  THE  ATMOSPHERE  ABOUT  THE  EARTH. 


CHAPTER    II. 

EXTENT   AND   ARRANGEMENT    OP   THE   ATMOSPHERE    ABOUT   THE   EARTH. 

13.  The  geosphere,  hydrosphere  and  atmosphere.  The  great  mass  of 
the  earth,  solid  at  least  in  its  outer  crust,  is  for  the  most  part  bathed  in  an 
ocean  of  water  and  is  entirely  surrounded  by  an  envelope  of  gases.  These 
three  parts  of  our  planet  are  sometimes  named  the  geosphere,  the  hydro- 
sphere, and  the  atmosphere;  the  latter  term  being  in  familiar  use,  the  others 
being  less  frequently  met.  The  arrangement  of  the  several  parts  will  be 
better  understood  if  we  recall  the  physical  properties  of  matter  in  the  three 
states,  solid,  liquid  and  gaseous. 

Solid  bodies  retain  a  definite  form  and  volume,  unless  acted  upon  by  some 
severe  strain.  The  solid  crust  of  the  earth  is  slowly  strained  and  crushed 
into  the  uneven  form  of  continents  and  mountains;  the  inequalities  thus 
acquired  would  be  retained  indefinitely,  if  it  were  not  for  the  slow  weathering 
and  washing  away  of  their  surface;  but  this  process  of  change  is  so  slow  that 
we  need  not  consider  it  further  in  the  study  of  meteorology.  For  our  pur- 
poses, the  geosphere  may  be  regarded  as  a  rigid  spheroid,  of  somewhat  irregu- 
lar surface. 

Liquids  tend  to  retain  a  definite  volume;  their  free  upper  surface  takes  a 
shape  at  right  angles  to  the  resultant  of  the  forces  acting  on  it,  while  their 
form  elsewhere  depends  on  that  of  the  body  upon  which  they  rest.  The 
liquid  hydrosphere  is  acted  on  by  the  centripetal  pull  of  terrestrial  gravitation; 
it  settles  down  in  the  depressions  of  the  earth's  crust,  forming  the  oceans, 
with  a  surface  standing  everywhere  at  right  angles  to  the  force  acting  on  it. 
If  terrestrial  gravitation  acted  alone,  the  surface  of  the  ocean  would  take  the 
shape  of  a  sphere;  for  a  sphere  alone  has  a  surface  everywhere  at  right  angles 
to  a  system  of  forces  directed  to  a  single  center:  but  on  account  of  the  centrif- 
ugal force  of  the  earth's  rotation,  the  ocean's  surface  is  deformed  into  a 
slightly  flattened  or  oblate  spheroid.1 

1  The  following  simple  statement  of  the  problem  may  serve  to  explain  the  deformation  of 
the  sphere  into  the  spheroid.  Inertia  is  the  resistance  that  a  body  opposes  to  a  force  that 
changes  the  velocity  or  direction  of  its  motion.  There  is  no  especial  name  given  to  that 
manifestation  of  inertia  which  comes  from  a  change  of  velocity;  but  the  inertia  developed 
by  a  change  in  the  direction  of  motion  is  called  by  the  special  name,  "centrifugal  force." 
It  is  in  no  proper  sense  a  force;  for  a  force  is  that  which  changes  or  tends  to  change  the 
direction  or  velocity  of  a  body's  motion.  That  form  of  inertia  resistance  which  is  called 
centrifugal  force  is  manifested  in  a  direction  opposite  to  that  of  the  actual  force  by  which 
the  body's  path  is  changed;  and  hence  in  the  case  of  a  circular  motion,  or  rotation,  in  which 


10 


ELEMENTARY   METEOROLOGY. 


The  level  surface  of  the  sea  is  naturally  taken  as  the  standard  of  reference 
in  measuring  the  heights  to  which  the  land  rises  above  it,  or  depths  to  which 
the  floor  of  the  ocean  basins  sink  below  it. 

Gases  do  not  retain  a  definite  form  or  volume.  They  continually  exert  an 
expansive  force,  tending  to  increase  their  volume.  If  acted  on  by  external 
forces,  they  may  be  compressed  into  smaller  and  smaller  volume;  if  free  to 
expand,  they  will  increase  in  volume  and  occupy  all  the  space  allowed  them. 
The  gases  of  the  atmosphere  are  drawn  down  on  the  surface  of  the  sea  and 
land  by  gravity;  the  weight  of  the  upper  strata  compresses  the  lower  ones 
into  greater  and  greater  density;  while  the  upper  ones  expand  to  an  extreme 
tenuity.  We  know  nothing  by  direct  observation  of  the  free  surface  of  a  gas; 
and  hence  cannot  well  understand  how  the  atmosphere  is  limited  upwards. 


14.     Dimensions  of  the  earth.     The  following  rough  table  of  dimensions 
is  of  interest  in  this  connection:  — 

Area  of  earth's  surface,    .......   197,000,000  square  miles. 

Volume  of  earth,     ........  256,000,000,000  cubic  miles. 

Mass  of  earth,     ..............     6xl021tons. 

Area  of  ocean,     .     .     150,000,000  square  miles,  or  f  of  earth's  surface. 
Volume  of  ocean,    ....  300,000,000  cubic  miles,  or  ^7  of  earth. 

Mass  of  ocean,    ........  13  x  1017  tons,  or  ^^  of  earth. 

Mass  of  atmosphere,    .....    5  x  1015  tons,  or  x^fffonro  of  earth. 


15.     Pressure  of  the  atmosphere.     The  level  surface  of  the  ocean  would 
everywhere  be  equally  pressed  upon  by  the  overlying  atmosphere,  if  there 

a  body  is  continually  pulled  by  a  centripetal  force  towards  a  center,  the  so-called  centrifugal 

force  is  manifested  outward  along  the  radius. 

In  the  case  of  any  part  of  the  ocean's  surface  layer,  A,  Fig.  1,  the  only  force  acting  on 

it  is  terrestrial  gravitation,  AG.     One  component  of  this  force,  AB,  must  be  expended  in 

overcoming  the  inertia  (centrifugal  force),  AC,  that 
arises  from  the  continual  change  in  the  direction  of  the 
body's  motion.  If  AB  is  expended,  only  the  other 
component,  Ag,  can  remain;  hence  it  is  only  the  latter 
component  of  terrestial  gravitation  which  acts  to  del  er- 
mine the  shape  of  the  ocean's  surface.  This  component 
is  called  gravity.  It  is  not  directed  to  the  earth's  cen- 
ter, but  slightly  away  from  the  center,  towards  the 
pole  of  the  opposite  hemisphere  from  that  in  which  A 
is  situated.  Consequently,  a  plumb  line  at  A  will  liaiiii 
in  the  direction  A<L  or  vertical;  a  water  surface  at  .1 
will  adjust  itself  at  ri.uht  angles  to  A(/,  or  level.  A 
continuous  ocean  surface  from  N  to  (}  will  everywhere 
adjust  itself  to  a  system  of  local  graviiativr  forces,  all 

of  which  are  turned  a  little  away  from  <)  towards  s.     Hence  NAQ  becomes  nA  q;  or  the 

sphere  is  changed  into  the  oblate  or  flattened  spheroid. 


ARRANGEMENT  OF  THE  ATMOSPHERE  ABOUT  THE  EARTH.     11 

were  no  disturbing  forces  present  by  which  a  greater  part  of  the  atmosphere 
might  be  accumulated  in  one  region  than  in  another.  The  surface  of  the  land 
suffers  a  less  and  less  pressure,  the  higher  it  rises  above  sea-level.  It  is  the 
pressure  of  the  atmosphere  on  the  water  in  a  well  that  raises  a  column  of 
water  in  the  tube  of  a  pump,  where  the  downward  pressure  of  the  air  is  re- 
moved by  raising  the  piston.  The  height  to  which  water  will  rise  in  a  pump 
may,  therefore,  be  used  to  determine  the  value  of  atmospheric  pressure.  This 
height  is  about  thirty-four  feet,  if  the  experiment  is  made  at  sea-level.  As 
the  weight  of  a  cubic  inch  of  water  is  0.036  of  a  pound,  the  pressure  of  the 
atmosphere  on  a  square  inch  of  surface  at  sea-level  must  be  14.7  pounds,  or 
about  a  ton  on  a  square  foot. 

16.  Barometers.  Water  is  not  heavy  enough  to  be  conveniently  used  in 
determining  atmospheric  pressure.  The  heavier  liquid,  mercury,  is  much  bet- 
ter adapted  to  this  purpose.  If  a  glass  tube,  about  thirty-two  inches  long, 
closed  at  one  end  and  filled  with  mercury,  be  inverted  and  the  open  end  placed 
in  a  dish  of  mercury,  the  height  at  which  the  mercury  will  then  be  held  in  the 
tube  affords  a  precise  and  convenient  indication  of  the  pressure  of  the  atmos- 
phere. Mercury  being  thirteen  and  a  half  times  heavier  than  water,  or 
10,784  times  heavier  than  air,  the  column  of  mercury  will  stand  at  a  height  of 
about  thirty  inches  at  sea-level;  but  the  length  of  the  mercury  column  will  be 
less  and  less  at  more  and  more  elevated  stations.  If  the  air  were  uniformly 
dense  at  all  altitudes,  the  upper  surface  of  the  atmosphere  would  be  found  at 
a  height  of  10,784  times  30  inches,  or  about  five  miles.  By  attaching  a 
scale  to  the  tube,  the  height  of  the  mercury  may  be  read,  and  thus  the 
barometer  or  pressure  measure  is  constructed.  It  is  customary  to  speak  of  the 
pressure  of  the  atmosphere  in  terms  of  barometric  inches;  that  is,  the  height 
in  inches  of  the  column  of  mercury  that  the  pressure  of  the  atmosphere  sus- 
tains in  the  tube  at  any  time.  The  precise  measure  of  what  is  called  one 
atmosphere  of  pressure  in  these  units  is  29.905;  the  mercury  having  a  tem- 
perature of  32°,  and  the  observations  being  reduced  to  the  latitude  of  London 
(see  Sect.  101) :  this  equals  a  pressure  of  760.00  mm.  at  latitude  45°.  Further 
account  of  the  construction  of  the  barometer  and  its  use  will  be  found  in 
Chapter  VI. 

A  thousand  cubic  feet  of  dry  air — that  is,  the  contents  of  a  cubic  room 
measuring  ten  feet  on  a  side — at  a  temperature  of  freezing  (32°  Fahrenheit) 
and  under  a  pressure  of  30  barometric  inches,  or  30  inches  of  mercury  as  it  is 
commonly  called,  weighs  75.29  pounds.  If  its  volume  be  kept  constant,  its 
expansive  force  will  increase  by  ?^5  of  the  expansive  force  at  freezing  for 
f-vpiy  rise  of  one  degree  Fahr.  in  temperature;  but  we  have  little  to  do  with 
this  condition  in  meteorology.  If,  as  is  much  more  commonly  the  case,  the 
pressure  upon  the  air  remains  constant,  it  will  expand  as  its  temperature  rises : 


12  ELEMENTARY    METEOROLOGY. 

its  volume  increasing  by  ? £n  or  0.002  of  the  volume  at  freezing  for  every  rise 
of  one  degree  Fahr.  (or  by  5}7  for  a  rise  of  one  degree  Centigrade). 

17.  Downward  pressure  of  the  ocean.     Return  now  to  the  case  of  the 
level  ocean,  on  which  the  atmosphere  everywhere  exerts  a  uniform  pressure. 
If   we   descend   about   33   feet   into  the  salt  waters  of   the   ocean,  we   may 
imagine  a  surface  there,    concentric    with    the    outer    spheroidal   surface   of 
the  ocean,  on  which  the  pressure  will  everywhere  be  two  atmospheres.     At  a 
depth  of  66  feet,  the  pressure  will  be  three  atmospheres,  and  so  on  to  the 
bottom.      Such    imaginary    surfaces    are    called    isobaric,  from   having   equal 
pressure  on  all  parts.     At  the  average  depth  of  the  ocean,  or  two  miles,  the 
pressure  will  be  320  atmospheres;  at  the  greater  depths  of  the  deepest  parts 
of   the  oceans,  between  four  and  five  miles,  the  pressure  rises  to  seven  or 
eight  hundred  atmospheres. 

Water,  however,  is  so  nearly  incompressible  that  even  under  these  enor- 
mous pressures,  its  density  is  not  greatly  increased.  The  water  at  the  bottom 
of  the  great  oceans  is  only  about  as  much  denser  than  the  surface  water,  as 
the  latter  is  denser  than  fresh  water.  Any  substance  that  is  heavy  enough  to 
sink  rapidly  below  the  surface  of  the  sea  will  sink  all  the  way  to  the  bottom. 

18.  Isobaric  surfaces  in  the  atmosphere.      The  case  of  the  atmosphere 
is  very  different.     It  is  highly  elastic,  and  hence  must  be  much  denser  at  the 
bottom  than  near  the  top.     If  we  ascend  about  900  feet  from  sea-level,  the 
barometric  pressure  there  will  be  reduced  from  30  to  29  inches.     At  this 
height  we  may  imagine  a  level  spheroidal  isobaric  surface,   concentric  with 
that  of  the  ocean,   on  which  the  pressure  of  the  overlying  atmosphere  is 
everywhere  29  inches.     How  high  must  one  ascend  to  reach  a  second  i.sobarie 
surface  on  which  the  pressure  would  be  28  inches  ?     If  the  height  of  the 
surface  of  29  inches  is  900  feet,  the  height  of  the  second  surface  above  the 
first  must  be  ff  of  900;  or  932  feet:   for  the  volumes  of  gases  are  known  to 
increase  as  the  pressure  by  which  they  are  confined  decreases.     The  height  of 
the  third  surface,  of  27  inches  pressure,  would  be  f  ^  of  900,  or  967  feet  above 
the  second  surface  ;  and  in  this  way  we  may  continue  to  calculate  the  altitude 
at  which  any  pressure  would  be  found.     The  pressure  of  *2(\  inches  would  thus 
be  found  at  a  height  of  3,800  feet  above  sea-level.     If  the  rule  were  followed 
to  the  last  case,  it  would  lead  us  to  say  that  the  height  at  which  the  surface  of 
no  pressure  —  that  is,  the  upper  surface  of  the  atmosphere  —  would  be  found, 
would  be  *$-  times  900  feet  above  the  surface  of  one  inch  pressure,  and  this 
would  be  at  an  infinite  distance  above  it;  but  it  is  not  probable  that  the  rule 
by  which  volumes  and  pressure  are  corn-Lite.!  can  be  fairly  applied  to  such 
extreme  cases.     Moreover,  the  known  decrease  of  temperature  in  the  upper  air 
would  somewhat  reduce  the  measures  here  given.     It  must  suffice  to  say  that 


ARRANGEMENT  OF  THE  ATMOSPHERE  ABOUT  THE  EARTH.     13 

the  air  becomes  thinner  and  thinner  as  we  ascend  above  sea-level ;  that  the 
successive  isobaric  surfaces  are  separated  by  greater  and  greater  distances ; 
but  of  the  absolute  termination  of  the  atmosphere  we  can  say  nothing. 

19.  Vertical  decrease  of   pressure  in  the  atmosphere.      It  is   possible, 
however,  to  calculate  with  a  close  approach  to  accuracy  the  height  at  which 
any  given  pressure  will  be  found  ;  or,  conversely,  the  pressure  corresponding 
to  any  given  height.     Allowance  must  be  made  in  this  calculation  for  the 
decrease  of  temperature  at  the  rate  of  1°  in  300  feet  in  ascending  above 
sea-level ;  and  for  certain  other  minor  corrections  ;  when  this  is  done,  we  find 
the  results  given  in  the  following  table  : — 

Pressure.  Altitv  le. 

30  inches.  ',/  feet 

29  910 

28  1,850 

27  2,820 

26  3,820 

25  4,850 

24  5,910 

23  7,010 

22  8,150 

21  9,330 

20  10,550 

18  13,170 

16  16,000 

20.  Height  of  the  atmosphere.     In  the  older  books  on  meteorology,  the 
height  stated  for  the  atmosphere  was  45  or  50  miles  ;  this  being  the  altitude 
at   which    no    significant    barometric    pressure   would  be  encountered ;    at  a 
height  of  30  miles  the  pressure  is  only  half  a  hundredth  of  an  inch.     Observa- 
tions on  the  duration  of  twilight  —  that  is,  of  the  perceptible  sunlight  which 
is  turned  from  its  direct  course  by  the  action  of  the  atmosphere  after  the  sun 
has  set  —  gave  about  the  same  measures  ;  but  this  depends  manifestly  upon  the 
delicacy  of  the  observations  by  which  the  duration  of  twilight  is  determined  ; 
if  our  eyes  were  sharper,  twilight  might  be  perceived  longer.     Hence  in  this 
case,  as  in  the  other,  all  that  can  be  said  is  that  at  a  height  of  about  50  miles, 
the  air  becomes  excessively  thin,  incapable  of  producing  significant  pressures, 
or  of  deflecting  perceptible  amounts  of  sunlight. 

Observations  of  meteors,  however,  give  much  greater  dimensions  for  the 
height  of  the  atmosphere.  Meteors  are  small  solid  bodies,  flying  rapidly 
through  space,  and  sometimes  entering  the  earth's  atmosphere.  Their  velocity 
is  so  great  that  they  generate  heat  enough  by  the  compression  of  the  air  in 


14  ELEMENTARY   METEOROLOGY. 

• 

their  path  to  render  them  luminous,  and  even  to  disintegrate  them  before  they 
reach  the  bottom  of  the  atmosphere.  Only  the  larger  ones  reach  the  ground 
unconsumed.  Meteors  have  sometimes  been  seen  from  several  places  by 
different  observers,  and  their  apparent  paths  among  the  stars  determined 
closely  enough  to  enable  one  afterwards  to  calculate  the  angular  altitude  of 
each  observer's  line  of  sight  above  the  horizon.  The  height  at  which  the  lines 
of  sight  intersect  may  then  be  easily  determined  ;  and  in  this  way  it  is  learned 
that  meteors  become  visible  at  heights  even  greater  than  100  miles.  Although 
it  is  difficult  to  conceive  of  the  excessive  tenuity  of  the  air  at  such  heights,  we 
are  constrained  to  believe  that  it  exists  there  ;  how  much  further  the  faintest 
traces  of  the  atmosphere  may  extend  must  be  left  to  future  discovery. 

Valuable  observations  of  meteors  may  be  made  by  any  persons  who  can 
record  the  time  of  appear  ir.ce  accurately  and  whose  knowledge  of  the  constel- 
lations is  sufficient  to  identify  the  stars  easily  ;  or,  even  without  this  knowledge, 
by  persons  who  will  take  care  to  notice  the  track  of  a  meteor  past  some  fixed 
objects,  whose  direction  and  angular  altitude  may  afterwards  be  measured  by 
surveyor's  instruments.  Such  observations,  reported  to  the  central  office  of 
any  of  our  state  weather  services,  may  be  compared  with  good  results.  A 
meteor  seen  over  New  England  on  September  6,  1886,  afid  reported  by  several 
observers  of  the  New  England  Meteorological  Society,  was  determined  by 
Professor  Newton  of  Yale  College  to  have  become  visible  at  an  altitude  of  90 
miles  over  northwestern  Vermont,  and  to  have  disappeared  at  an  altitude  of 
25  miles  over  southeastern  New  Hampshire. 

We  know  little  of  the  vast  upward  expanse  of  the  atmosphere.  The  rays 
from  the  sun  and  stars  enter  through  it ;  hence  it  must  be  excessi  vely  thin 
and  pure.  Meteors  dart  into  it,  and,  if  heavier  than  a  few  ounces  weight,  in 
most  cases  fall  to  the  earth.  But  the  highest  mountains  do  not  rise  six  miles 
above  sea-level,  and  their  upper  slopes  are  deserts  of  rock  and  snow.  The 
highest  flight  of  a  balloon,  nearly  seven  miles,  reached  temperatures  so  low 
and  air  so  thin  that  the  balloonists  fainted.  All  the  clouds  of  those  lofty 
regions  consist  of  minute  ice  crystals  ;  but  they  are  seldom,  if  'ever,  measured 
at  heights  greater  than  eight  or  nine  miles.  It  is  only  the  lower  part  of  the 
atmosphere  that  is  of  sufficient  density  and  of  a  high  enough  temperature  for 
the  easy  maintenance  of  life,  as  it  has  been  developed  on  the  earth.  The 
greater  density  of  the  lower  strata  has  already  been  explained  as  a  result  from 
the  pressure  of  the  upper  strata  ;  we  may  next  inquire  as  to  the  control  of  the 
temperature  of  the  atmosphere. 


CONTROL  OF  ATMOSPHERIC  TEMPERATURES  BY  THE  SUN.      15 


CHAPTER  III. 

THE  CONTROL,  OF  ATMOSPHERIC  TEMPERATURES  BY  THE  SUN. 

21.  Sources  of  heat.     The    only  sources  of  heat  on  which  the  tempera- 
ture of  the  atmosphere  may  be  conceived  to  depend  are  the  sun,  the  stars  and 
the  earth. 

The  earth '  is  still  very  hot  within ;  but  its  hot  interior  mass  is  so  well 
sealed  over  by  a  non-conducting  crust,  that  very  little  heat  escapes  outward 
through  it.  Volcanic  eruptions  occasionally  carry  some  of  the  glowing  rocks 
from  the  interior  up  to  the  surface  in  a  spasmodic  manner;  but  these  eruptions 
are  manifestly  too  exceptional  for  further  mention.  Moreover,  if  the  tempera- 
ture of  the  air  over  the  earth's  surface  depended  on  conduction  from  the 
's  interior,  we  should  not  know  ho\y  to  explain  the  changes  of  tempera- 
ture  from  the  equator^to  the  poles,  or  from  summer  to  winter. 
>  ^-\"The  stars  are  as  hot  as  the  sun,  and  they  are  innumerable ;  but  their  dis- 
tance is  so  great  that  they  control  our  temperature  as  little  as  they  control  our 
light.  Moreover,  they  shine  from  all  parts  of  the  sky;  hence,  if  our  tempera- 
ture were  said  to  depend  on  star  beams,  we  should  not  know  how  to  explain 
the  changes  of  temperature  from  day  to  night. 

The  sun  is  manifestly  the  ruler  of  temperatures  on  the  earth's  surface. 
The  changes  of  temperature  from  equator  to  pole,  from  summer  to  winter, 
from  day  to  night,  all  follow  the  changes  in  the  intensity  of  sunshine.  There 
cuii  be  110  doubt  in  an  explanation  where  variations  in  the  effects  follow  so  pre- 
cisely the  variations  in  their  presumed  cause. 

When  we  come  to  explain  how  it  is  that  the  sun  controls  terrestrial  tem- 
perature, it  is  necessary  to  go  slowly,  if  we  would  gain  a  clear  understanding 
of  the  process. 

22.  Nature  of  heat.     It  must  be  now  recalled  from  the  study  of  physics, 
that  heat  is  not  a  thing  in  itself,  but  simply  the  energy  of  the  molecular  mo 
tion  of  any  material  substance.     If  two  masses  of  lead  be  struck  violently 
together,  they  become  hotter  than  before.     Iron  and  steel  are  not  so  well  fitted 
for  this  experiment,  for,  by  reason  of  their  elasticity,  their  energy  of  impact 
is  mostly  expended  in  producing  a  rebound.     Lead  being  relatively  inelastic,  it 
is  supposed  that  the  energy  of  the. impact  is  expended  in  exciting  the  mole- 
cules of  which  the  masses  are  composed  to  a  more  active  motion;    and  we 
recognize  this  more  active  motion  in  the  higher  temperature  of  the  bodies. 
Tt  must  be  clearly  understood  that,  in  speaking  of  molecules,  or  of  heat  as  the 


16  ELEMENTARY    METEOROLOGY. 

energy  of  molecular  motion,  we  are  speaking  of  things  that  have  never  been 
seen;  of  phenomena  that  have  never  been  observed.  It  is,  nevertheless, 
reasonable  to  believe  in  molecules  and  in  the  mechanical  theory  of  heat,  as  it 
is  called,  because  by  the  acceptance  of  such  hypotheses,  we  are  enabled  to 
explain  and  correlate  a  great  variety  of  well  ascertained  facts,  that  have  other- 
wise found  no  explanation. 

Heat,  then,  is  believed  for  good  reasons  to  be  the  energy  of  molecular  mo- 
tion. When  the  surface  of  the  ground  and  the  lower  layers  of  air  become 
warmer  under  the  rays  of  the  morning  sun,  we  believe  that  their  molecules 
have  been  excited  to  greater  velocity  of  movement:  but  then  the  question 
arises  —  how  can  the  velocity  of  their  molecules  be  affected  by  the  sun,  which 
is  distant  from  the  earth  by  ninety-two  million  miles  !  The  space  between 
the  planets  and  the  sun  must  be  empty;  otherwise,  the  planets  could  not 
maintain  a  constant  distance  from  the  sun;  they  would  approach  it  on  an  in- 
ward spiral  path,  moving  faster  as  they  neared  it,  and  thus  accomplishing  an 
annual  revolution  in  a  decreasing  number  of  days.  This  is  not  the  case,  as 
far  as  observations  go.  We  must,  therefore,  suppose  space  to  be  essentially 
free  of  resisting  matter;  if  any  medium  is  there  for  the  propagation  of  energy 
from  one  mass  to  another,  it  must  be  of  properties  quite  unlike  those  of  the 
gross  forms  of  matter  that  we  know  on  the  earth,  rarer  even  than  the  ex- 
tremely tenuous  upper  strata  of  the  atmosphere.  Yet  some  continuous 
medium  must  be  supposed  to  exist  all  through  space;  for  without  it  we  cannot 
advance  towards  an  understanding  of  the  process  by  which  the  light  of  the 
stars  reaches  us;  or  by  which  the  sun  controls  our  temperature;  or  by  which 
a  hot  ball  of  iron  can  become  cool  even  though  suspended  in  a  vacuum. 

23.  Explanation  by  hypothesis.  Here,  as  in  the  mechanical  theory  of 
heat,  we  must  have  recourse  to  some  hypothesis;  and  our  faith  in  the  hypothe- 
sis should  be  measured  simply  by  its  success  in  explaining  facts  of  observation. 
It  is  commonly  the  case  in  the  progress  of  science  that  various  hypotheses  are 
successively  advanced  in  the  attempt  to  explain  facts  of  observation;  the 
hypothesis  which  best  accounts  for  all  the  facts  will  come,  in  time,  to  be  gen- 
erally accepted.  For  example,  many  facts  are  known  concerning  the  motions 
of  bodies;  the  falling  of  any  object  towards  the  earth;  the  movements  of  the 
moons  around  the  planets,  and  of  the  planets  around  the  sun;  and  even  the 
revolution  of  the  two  components  of  a  double  star  around  a  common  center. 
All  these  facts  of  motion  may  be  explained  by  the  hypothesis  that  every  mass 
of  matter  attracts  every  other  mass  with  a  force  directly  proportional  to  the 
product  of  their  masses,  and  inversely  proportional  to  the  square  of  the  dis- 
tance between  the  two.  This  hypothesis  is  so  successful  in  explaining  all 
relevant  facts,  that  it  has  come  to  be  universally  accepted.  The  force  of 
attraction  is  called  gravitation  ;  the  statement  of  the  manner  in  which  it 


CONTROL  OF  ATMOSPHERIC  TEMPERATURES  BY  THE  SUN.      17 

varies  is  called  the  law  of  gravitation.     The  discovery  and  establishment  of 
this  law  is  the  chief  glory  of  the  immortal  Newton. 

The  hypothesis  by  which  we  seek  to  explain  the  loss  of  heat  from  a  hot 
ball  suspended  in  a  vacuum,  or  from  the  sun  standing  in  empty  space, 
must  now  be  briefly  outlined.  It  may  be  named  the  hypothesis  of  radiant 
energy. 

24.  Radiant  energy.  It  has  been  supposed  that  in  spite  of  the  apparent 
emptiness  of  space,  as  far  as  molecular  matter  is  concerned,  it  is  nevertheless 
filled  with  an  all-pervading  medium  of  perfect  continuity,  excessive  rarity  and 
extraordinary  elasticity.  Any  disturbance,  such  as  the  agitation  of  the  mole- 
cules of  ordinary  matter,  at  once  imparts  a  disturbance  to  the  imagined 
medium;  and  then,  much  in  the  same  way  as  waves  spread  out  from  a  point 
where  a  stone  falls  into  a  body  of  water,  so  waves  of  disturbance  spread  out 
or  radiate  through  the  imagined  medium  in  all  directions  from  the  center  of 
excitement. 

Whether  the  hypothetical  medium  be  named  the  luminiferous  ether,  or 
whether  it  takes  a  name  from  its  property  of  propagating  electro-magnetic 
disturbances,  this  matters  nothing  to  us  now  ;  all  that  the  hypothesis  of 
radiant  energy  demands  is  that  molecular  disturbance  should  spread  away 
or  radiate  by  a  wave-like  motion  or  undulation  in  some  space-pervading 
medium.  Hence  the  name,  undulatory  hypothesis,  originally  employed,  when 
the  idea  of  the  undulatory  nature  of  light  was  first  introduced  by  Huyghens  in 
the  seventeenth  century,  in  distinction  to  the  corpuscular  hypothesis  of 
Newton,  which  assumed  that  the  light  from  the  sun  consisted  of  a  shower  of 
minute  corpuscles. 

The  undulations  of  radiant  energy  travel  away  in  straight  lines  in  all 
directions  from  their  source  at  the  enormous  velocity  of  nearly  two  hundred 
thousand  miles  a  second.  They  require  only  about  eight  minutes  to  span  the 
space  from  the  sun  to  the  earth.  The  dimensions  of  the  undulations  are 
excessively  minute  ;  those  given  out  from  the  sun  varying  in  length  between 
0.00270  and  0.00029  millimeter,  or  0.00011  and  0.00001  inch.  Their  undulal 
tion  is  incredibly  rapid  ;  reaching  several  hundred  million  million  undulations 
in  a  second,  the  finer  waves  swinging  the  more  quickly.  They  may  be  likened 
to  the  much  larger  waves  of  sound  excited  in  the  atmosphere  by  an  orchestra. 
The  trombones  and  bassoons  produce  long  waves  of  relatively 'slow  undula- 
tion ;  the  fifes  and  the  high  notes  of  the  violins  excite  finer  waves  of  relativel}r 
rapid  undulation  ;  the  louder  notes  excite  waves  of  greater  breadth  of  swing,  or 
amplitude  ;  but  they  all  travel  away  at  a  uniform  velocity.  On  encountering 
any  object,  they  spend  their  energy  upon  it ;  that  is,  they  set  it  into  vibration; 
thus  the  waves  from  one  tuning-fork  may  set  up  vibrations  in  another.  If  the 
waves  of  sound  disturb  the  delicate  mechanism  of  the  ear,  they  give  us  the 


18  ELEMENTARY    METEOROLOGY. 

sensation  of  hearing.  In  a  very  similar  way,  the  waves  of  radiant  energy, 
varying  in  period  and  amplitude  of  undulation,  move  on  from  any  exciting 
source  in  straight  lines  or  rays  at  a  uniform  velocity  as  long  as  they  pass 
through  what  we  call  empty  space  ;  but  if  the  waves  encounter  sensible 
matter,  some  of  the  energy  of  undulations  is  imparted  to  its  molecules,  and 
the  velocity  of  molecular  movement  is  increased  ;  that  is,  the  mass  that  the 
molecules  constitute  is  heated.  A  careful  distinction  should  be  drawn  between 
radiant  energy,  whose  nature  is  essentially  undulatory,  and  molecular  energy 
or  heat,  which  is  characterized  by  a  confused  molecular  agitation. 

25.  Radiation  from  the  sun  :  insolation.  The  sun  is  an  enormous  globe  of 
excessively  hot  matter  ;  many  times  hotter  at  its  cloudy  surface  than  the  hot- 
test furnace,  and  presumably  hotter  still  within.  Its  mass  is  so  great  that  in 
spite  of  its  active  radiation  of  energy,  by  which  its  heat  is  reduced,  it  will 
be  hot  for  ages  to  come.  Its  diameter  is  about  880,000  miles  ;  if  the  earth 
were  at  the  center  of  the  sun,  and  the  moon  were  revolving  about  the  earth  at 
its  present  distance  of  240,000  miles,  there  would  still  be  a  solar  shell  outside 
of  the  moon  200,000  miles  thick. 

The  radiant  energy  or  radiation  emitted  by  the  sun  is  conveniently  given 
the  special  name  of  insolation.  It  varies  greatly  in  wave-length  and  period  of 
undulation.  It  flies  away  in  all  directions  at  an  incredible  velocity.  The 
greater  part  of  it  goes  on  and  on  for  ages  through  the  void  of  the  universe, 
constantly  becoming  fainter  as  it  embraces  wider  and  wider  spheres  of  action  ; 
only  a  minute  part  of  the  emitted  insolation  encounters  a  planet  on  its  way 
through  space. 

The  heat  emitted  from  a  small  area  of  the  sun's  surface  has  been  compared 
with  that  given  out  from  an  equal  area  of  melted  steel  in  a  Bessemer  furnace  ; 
the  ratio  being  87  to  1  in  favor  of  the  sun.  The  heat  received  from  the  sun's 
rays  falling  vertically  and  unobstructed  on  a  square  mile  of  the  earth's  surface 
would  warm  750  tons  of  water  from  the  freezing  to  the  boiling  point  in  a 
minute.  The  whole  amount  of  heat  received  from  the  sun  on  the  earth  in  a 
minute  would  warm  37,000,000,000  tons  of  water  by  the  same  amount.  The 
heat  thus  received  would  suffice  to  melt  a  layer  of  ice  about  160  feet  thick  »\  cr 
the  whole  earth  in  a  year  ;  this  is  several  thousandfold  greater  than  that 
received  from  the  earth's  interior  ;  and  yet  the  earth  receives  only  one  two- 
billionth  part  of  .the  heat  given  out  by  the  sun  !  The  other  planets  receive 
similar  minute  fractions  ;  the  rest  is  "wasted"  ;  that  is,  we  do  not  see  that 
it  is  applied  to  any  particular  purpose.  Upon  the  trifling  share  of  insolation 
received  by  the  earth  depend  all  our  activities,  except  those  of  telluric  origin, 
such  as  earthquakes  and  volcanoes  ;  and  those  of  lunar  origin,  such  as  th^ 
oceanic  tides.  The  winds,  the  ocean  currents  and  all  the  processes  of  life 
depend  on  energy  received  from  the  sun. 


CONTROL  OF  ATMOSPHERIC  TEMPERATURES  BY  THE  SUN.      19 

Whatever  one  may  feel  about  the  correctness  of  the  undulatory  theory 
of  radiant  energy  and  however  difficult  it  may  be  to  grasp  its  fundamental 
conditions,  it  must  be  recognized  as  more  nearly  explaining  the  facts  witli 
which  it  is  concerned  than  any  other  theory  that  has  been  proposed.  Its 
essential  feature  of  undulation  is  universally  accepted  by  physicists,  althougn 
the  nature  of  the  medium  by  which  the  undulations  are  transmitted  is  by  no 
means  understood. 

26.  Astronomical  relations  of  sun  and  earth.     Having  now  considered 
the  method  by  which  the  sun's  heat  is  transformed  into  radiant  energy  ancl 
thus  propagated  outward  in  all  directions,  so  that  the  earth  is  constantly  in 
receipt  of  a  small  fraction  of  the  total,  we  must  next  examine  the  position  of 
the  earth  with  respect  to  the  sun  at  different  times  of  the  year,  so  as  to  under- 
stand clearly  how  the  incident  insolation  is  distributed  over  its  surface  ;  for 
on  this  distribution  depends  the  division  of  the  earth  into  zones  and  all  the 
changes  of  the  seasons. 

The  earth  moves  around  the  sun  in  a  nearly  circular  orbit.  The  actual, 
form  of  the  orbit  is  an  ellipse,  but  of  so  faint  eccentricity  that,  the  sun  being 
in  one  focus,  the  earth  is  only  three  million  miles  nearer  the  sun  at  one  time 
than  another.  The  time  of  .nearest  approach,  called  perihelion,  occurs  con^ 
veniently  for  our  memory  on  New  Year's  Day  ;  and  the  time  of  greatest 
distance,  called  aphelion,  on  July  1.  It  may  be  seen  at  once  from  this  that 
it  cannot  be  on  our  distance  from  the  sun  that  the  winter  and  summer  of  the 
northern  hemisphere  depend.^ 

27.  Distribution  of  insolation  over  the  earth.     The  axis  of  the  earth,  on 
which  it  turns  once  a  day,  does  not  stand  vertical  to  the  plane  of  its  orbit ; 
but  is  inclined  twenty -three  and  a  half  degrees  from  the  vertical,  and  in  such' 
a  direction  as  to  turn  the  north  pole  away  from  the  sun  on  the  21st  of  Decem- 
ber ;  that  is,  ten  days  before  the  time  of  perihelion.     As  the  earth  moves 
around  the  orbit,  the  axis  always  stands  parallel  to  itself ;  hence  on  June  20, 
the  north  pole  will  be  turned  toward  the  sun.     These  dates  are  called  the 
solstices,  because  the  sun  then  stands  farthest  south  or  north  of  the  plane  of 
the  earth's  equator.     It  follows   from  this   that   the   amount   of   insolation 
received  at  different  latitudes  will  vary  greatly  during  the  year  ;  first,  because 
the  inclination  of  the  sun's  rays  to  the  horizon  varies  ;  second,  because  the 
diurnal  duration  of  sunshine,  or  the  part  of  the  twenty-four  hours  in  which 
the  sun  stands   above  the  horizon,  varies.     The  change  of  seasons  is  thus 
determined. 

The  noon  altitude  of  the  sun  and  the  length  of  the  day  at  any  latitude  are 
best  illustrated  by  fitting  a  paper  ring  around  a  globe  in  the  attitude  of  a 
great  circle;  the  globe,  with  the  axis  properly  tilted,  being  carried  around  a 


ELEMENTA It V    M ETEOKOLOG V. 

curve  to  represent  the  orbit,  and  the  paper  ring  being  always  adjusted  at  right 
angles  to  a  line  from  an  imaginary  sun  within  the  orbit.  The  ring  will  then 
separate  the  light  and  dark,  or  day  and  night  halves  of  the  earth,  and  may 
therefore  be  called  the  twilight  circle.  If  a  line  is  drawn  through  the  sun  at 
right  angles  to  the  solsticial  line,  it  will  intersect  the  orbit  at  two  points;  and 
while  the  earth  occupies  either  of  these  points,  the  twilight  circle  passes 
through  the  poles,  and  the  days  and  nights  are  everywhere  equal.  The  points 
are,  therefore,  called  the  equinoxes,  and  are  passed  on  March  20  and  Septem- 
ber 22.  It  should  be  noted  that  the  line  denning  the  equinoxes  does  not  cut 
the  orbit  in  halves,  and  that  seven  days  longer  time  is  spent  in  passing  from  the 
vernal  equinox  through  aphelion  to  the  autumnal  equinox  than  from  the 
autumnal  through  perihelion  to  the  vernal. 

At  other  times  than  the  equinoxes,  day  and  night  are  unequal,  because  the 
twilight  circle  then  cuts  the  latitude  circles  unsy  in  metrically.  This  effect  is 
strongest  at  the  solstices,  when  the  twilight  circle  is  most  oblique  to  the 
latitude  circles;  but  the  equator  always  has  equal  days  and  nights,  because, 
being  a  great  circle,  it  muFJ  always  be  bisected  by  the  twilight  circle.  A  little 
practice  with  a  globe  should  make  this  plain. 

The  altitude  of  the  sun  over  an  observer's  horizon  is  always  greatest  at 
noon.  The  noon  altitude  will  be  90°  at  some  point  within  the  tropics  through- 
out the  year ;  elsewhere  even  the  noon  rays  fall  obliquely,  and  their  effect 
weakens  as  they  spread  over  a  greater  surface  than  their  cross  section.  This 
may  be  likened  to  the  noon  sunshine  in  winter  on  a  north  and  south  road 
passing  over  a  hill;  the  snow  on  the  southern  slope,  receiving  the  insolation 
more  nearly  at  right  angles  to  the  surface,  may  be  melted,  while  it  remains 
frozen  on  the  northern  slope,  where  the  insolation  falls  more  obliquely. 

The  proportionate  amounts  of  insolation  received  in  a  single  day  at  differ- 
ent latitudes  and  at  different  times  of  the  year  have  been  carefully  calculated, 
and  are  presented  in  abstract  in  the  following  table.  The  unit  of  these  meas- 
ures is  the  amount  of  insolation  received  at  the  equator  on  the  day  of  the 
vernal  equinox,  March  20,  when  the  sun  passes  through  the  plane  of  the 
earth's  equator  on  its  way  into  the  northern  hemisphere  of  the  sky. 


Latitude 

0° 

-1-  20° 

-f-40° 

+  60° 

+  90° 

—  90° 

March  20     

1.000 

0.934 

0.763 

0499 

0000 

0  000 

June  21  

0.881 

1040 

1.103 

1.090 

1.202 

o.ooo 

September  22  ... 

0  »K1 

n  J»:;K 

0  760 

0  499 

0  000 

0  000 

December  21     

0.042 

0679 

0.352 

0000 

0.000 

1    I'Sl 

Annual  Total    .... 

347 

329 

274 

197 

143 

143 

The  same  data  are  also  presented  in  graphic  form  in   Kij,r.  2;  the  latitude 
being  given  on  the  left  margin,  the  time  of  year  on  the  right  margin,  and  the 


CONTROL  OF  ATMOSPHERIC  TEMPERATURES  BY  THE  SUN. 


21 


value  of  insolation  being  indicated  by  a  vertical  measure  from  the  plane  of 
the  two  margins. 

The  most  important  lessons  of  the  table  and  diagram  are  as  follows: — - 
(1)  On  the  equinoxes,  the  greatest  amount  of  insolation  is  received  at  the 
equator,  where  the  day  is  twelve  hours  long  and  the  sun  passes  through  the 
zenith  at  noon;  on  this  day  at  higher  latitudes  in  either  hemisphere,  although 
the  day  is  still  twelve  hours  long,  the  sun  does  not  reach  the  zenith  and  the 
value  of  insolation  progressively  diminishes;  at  the  poles,  where  the  rays  pass 
tangent  to  the  surface  of  the  earth  (except  for  a  slight  bending  by  atmos- 
pheric refraction),  no  insolation  is  received.  (2)  On  our  summer  solstice, 


FIG.  2. 

June  21,  the  equator  still  has  a  day  twelve  hours  long,  but  the  sun  does  no^i 
reach  the  zenith  then,  and  hence  less  insolation  is  received  there  on  this  dateV 
than  on  the  equinoxes.     Southern  latitudes  have  still  more  oblique  sunshine^ 
and  a  shorter  and  shorter  day,  until  beyond  latitude  66^°  the  night  is  twenty^ 
four  hours  long,  and  the  insolation  for  the  whole  southern  frigid  zone  is  zeroV 
North  of  the  equator,  the  sun  reaches  the  zenith  over  latitude  23^°,  and  as  the)* 
day  is  there  more  than  twelve  hours  long,  the  amount  of  insolation  received^ 
at  this  latitude  on  the  solstice  is  greater  than  that  received  at  the  equator  on 
the  equinox.     Going  further  north,  there  is  for  a  time  an  increase  in  the  value 
of  the  diurnal  insolation,  because  the  loss  from  the  lower  noon  altitude  of  the 


22  ELEMENTARY    METEOROLOGY. 

sun  is  overcome  by  the  gain  from  the  greater  length  of  the  day ,  a  maximum 
value  being  reached  about  latitude  40°;  then  tl  ere  is  a  slow  weakening,  as  the 
decreasing  noon  altitude  more  than  compensates  for  the  continued  gain  in  the 
length  of  the  day,  the  minimum  being  found  just  beyond  latitude  60°;  from 
this  latitude,  northward,  the  increasing  length  of  the  day  and  the  presence 
of.  the  sun  above  the  horizon  during  the  whole  twenty-four  hours  cause  an  in- 
crease in  the  value  of  diurnal  insolation,  reaching  a  strong  maximum  at  the 
pole,  where  the  amount  received  is  decidedly  greater  than  at  the  equator  on 
any  day  of  the  year.  (3)  On  our  winter  solstice,  December  21,  the  relations 
of  the  northern  and  southern  hemispheres  are  inverted;  and  the  south  pole 
then  receives  even  a  greater  measure  of  insolation  than  the  north  pole  received 
six  months  before,  because  the  earth  is  then  near  perihelion.  The  limits  of 
the  zones  are  thus  seen  to  be  related  to  the  distribution  of  insolation. 

It  should  be  noted  that  the  distribution  of  insolation  depends  largely  on 
the  length  of  the  day  as  well  as  on  the  altitude  reached  by  the  sun.  It  also 
appears  that  the  known  distribution  of  temperature  on  the  earth  does  not 
closely  follow  the  distribution  of  insolation;  for,  if  so,  the  north  pole  should 
have  a  relatively  high  temperature  on  June  21 ;  and  the  south  pole  should 
have  even  a  higher  temperature  on  December  21.  The  explanation  of  this 
discrepancy  will  be  found  in  Section  91. 

28.  Action  of  insolation  on  the  earth.     An  important  step  may  now  be 
made  in  considering  the  action  of  insolation  on  the  earth.     We  have  learned 
the  nature  of  this  form  of  energy;  we  have  seen  its  distribution  over  the 
earth  at  different  seasons;  the  effects  that  it  produces  come  next  in  order. 

It  must  be  carefully  borne  in  mind  that  radiant  energy,  while  on  its  way 
from  the  sun  to  the  earth,  is  not  heat.  It  was  excited  by  the  heat  of  the  sun, 
where  it  was  emitted;  and  it  will  produce  heat  when  its  energy  is  acquired  or 
absorbed  by  the  substances  of  the  earth;  but  until  thus  absorbed,  it  must  be 
regarded  only  as  a  special  form  of  energy  ;  a  shower  of  almost  immeasurably 
rapid  undulations,  varying  in  period  and  amplitude,  but  constant  in  velocity 
of  propagation. 

29.  Reflection.     When  radiant  energy  from  any  source  encounters  sensible 
mutter,  it  may  be  turned  b;u-k  or  reflected;  it  maybe  passed  on  or  transmitted ; 
or  it  may  be  expended  in  adding  to  the  molecular  energy  of  the  body;  that  is, 
the  nirriry  is  absorbed  and  the  body  is  heated.     Burnished  silver  is  the  best 
reflector  known;   it  turns  back  nearly  98  per  cent,  of  all  rays  incident  upon  it: 
and  this  in  a  most    >  \Mcmatio  manner,  BO  that  the  angle  of  reflection  equals 
the  an.u'le  of  incidence.     The  reflected  rays  depart  without  loss  of  energy,  and 
the  reflecting  body  is  not  warmed  by  them.     A  good  reflector  can   neither  be 
heated  easily  by  absorption,  nor  be  cooled  easily  by  its  own  radiation. 


CONTROL  OF  ATMOSPHERIC  TEMPKKATTKES  BY  THE  SIN.      23 

Snow  and  water,  whether  on  the  surface  of  the  earth  or  in  the  clouds,  are 
among  the  best  natural  reflectors;  much  of  the  insolation  that  falls  on  them 
is  turned  back  into  space,  and  the  earth  gains  nothing  from  it. 

30.  Transmission.      Rock  salt  is  the  best  transmitter  of  all  solid  sub- 
stances.    It  is  much  better  in  this  respect  than  glass,  which  absorbs  many  of 
the  finer  and  coarser  waves.     Such  a  substance  is  said  to  be  diathermanous,  or 
to  possess  the  quality  of  diathermance  ;  these  terms,  literally  meaning  "  open 
to  heat,"  having  been  introduced  when  radiant  energy  and  heat  were  con- 
founded.    The  temperature   of  a  transmitter  is  unchanged   by  the   radiant 
energy  that  passes  through  it.     It  can  be  warmed  only  by  the  energy  of  the 
few  waves  that  it  absorbs  ;  hence  like  a  reflector  it  must  warm  slowly,  even  in 
the  full  glare  of  sunshine. 

The  gases  of  the  atmosphere  are  almost  perfect  transmitters.  Any  given 
thin  layer  of  air  retains  very  little  of  the  insolation  incident  upon  it ;  it 
reflects  none  ;  nearly  all  passes  through  it.  It  can,  therefore,  warm  very 
slowly.  The  pure  water  of  the  ocean  is  also  a  comparatively  good  transmit- 
ter ;  but  water  is  much  denser  than  air,  and  the  sun's  rays  are  so  weakened 
by  the  slight  absorption  of  successive  layers  of  water,  every  one  of  which 
takes  a  little  of  the  passing  energy,  that  at  greater  depths  than  a  few  hundred 
fathoms,  sunshine  must  be  practically  imperceptible.  It  is  curious  to  discover 
in  this  connection  that  many  animals  inhabiting  the  deeper  parts  of  the  ocean 
have  well  developed  eyes,  and  are  brightly  colored  ;  and  hence  we  must 
suppose  that  light  from  some  source  reaches  them.  Animals  dwelling  in  dark 
caverns  do  not  develop  their  eyes,  and  are  white  or  gray  ;  hence  caverns  must 
be  darker  than  the  bottom  of  the  ocean.  The  phosphorescence  of  many 
marine  animals  may  serve  to  illuminate  the  ocean  bottom. 

31.  Absorption.     Carbon  is  an  almost  perfect  absorber.     It  reflects  only  £•' 
trifle  of  incident  insolation  and  absorbs  the  rest ;    it  is,  therefore,  rapidly : 
heated.     The  surface  of  the  greater  part  of  the  land,  being  a  poor  reflector 
of  radiant  energy  and  transmitting  none  beneath  the  surface,  absorbs  nearly  all  : 
that  falls  on  it,  and  warms  rapidly  under  sunshine. 

32.  Various  effects  of  absorption.     A  brief  digression  must  be  made  here> 
^to  refer  to  certain  other  effects  produced  by  the  absorption  of  insolation.     1$ 
ithe  solar  waves  fall  on  certain  substances,  such  as  those  used  on  photographic): 
^plates,  chemical  changes  of  composition  follow  ;  but  these  changes  are  not  to> 
^be  regarded  as  the  work  of  the  absorbed  rays.     Substances  thus  affected  may> 
=^be   regarded  as   so   many  springs,  bent  by  previous  chemical  reactions  into> 
^constrained  positions,  from  which  they  would  gladly  free  themselves  if  they> 
<pould  but  make  a  beginning.     The  excitement  caused  among  the  molecules  by> 
<(the  absorption  of  sunshine  or  of  radiant  energy  from  any  other  sufficient^ 


-4  ELEMKNTAKV    M  KTK<  >R(  >LOG  Y. 

source  merely  releases  the  springs  ;  and  the  consequent  rearrangement  of 
composition  is  the  work  of  chemical  attractions  then  allowed  to  operate. 
Certain  wave-lengths  are  more  effective  than  others  in  touching  off  certain 
substances.  In  ordinary  photographic  processes,  where  salts  of  silver  are 
employed,  the  coarser  solar  rays  have  little  effect ;  the  finer  rays  are  most 
useful  ;  and  these  are  for  this  reason  sometimes  called  actinic  rays.  Sometimes 
other  substances  are  employed,  on  which  the  coarser  waves  are  more  active. 

Most  animals  have  developed  special  organs,  which  we  call  eyes,  that  are 
particularly  sensitive  to  the  action  of  radiant  energy  of  certain  wave-lengths. 
In  the  human  eye,  no  effect  is  produced  by  waves  of  greater  length  than 
0.00075  mm.  or  of  less  length  than  0.00036  mm.;  but  rays  of  intermediate 
wave-lengths,  if  of  sufficient  strength  or  amplitude,  produce  a  sensation  which 
wt«  call  light.  If  the  coarser  waves  preponderate,  we  call  the  light  red  ;  if  the 
finer  ones  are  the  stronger,  we  call  the  light  blue  ;  the  intermediate  ones  cor- 
responding to  the  various  colors  of  the  spectrum.  Rays  perceptible  by  the 
eye  are  therefore  called  rays  of  light,  or  optical  rays. 

As  has  already  been  stated,  when  rays  of  any  wave-length  are  absorbed  by 
ordinary  substances,  heat  is  produced.  This  is  true  of  rays  of  much  coarser 
wave-length  than  can  be  perceived  by  the  eye  ;  hence,  it  has  been  common  to 
speak  of  such  as  "  dark  heat  rays." 

It  has  happened  that  the  progress  of  science  has  been  in  an  order  almost 
the  opposite  of  that  in  which  the  subject  of  radiant  energy  is  here  presented. 
The  radiation  of  light  was  first  studied  ;  then  "  radiant  heat  "  was  investigated. 
It  was  the  custom  for  a  time  to  name  the  different  kinds  of  solar  undulations 
after  the  effects  that  they  produced  ;  those  of  intermediate  length  were  called 
"light  rays"  ;  the  finer  ones,  "chemical  or  actinic  rays"  ;  the  coarser  ones, 
"heat  rays."  With  this  nomenclature,  it  was  implied  that  the  differences 
between  light,  heat  and  chemical  action  were  inherent  in  the  rays  themselves. 
As  now  understood,  the  differences  depend  on  the  nature  of  the  substance  by 
which  the  rays  are  absorbed.  For  this  reason,  no  proper  appreciation  of  the 
dependence  of  the  earth's  temperature  on  solar  energy  can  be  gained  until  it  is 
clearly  understood  that,  while  the  rays  are  on  the  way  across  the  emptiness  of 
space,  their  waves  differ  only  in  length,  in  period  of  undulation  and  in  ampli- 
tude. None  of  the  rays  should  in  any  proper  sense  be  then  called  light  rays, 
heat  rays  or  chemical  rays.  They  should  be  named  only  according  to  their 
inherent  peculiarities,  namely,  their  length  or  their  period  of  vibration  ;  and 
not  according  to  their  variable  effects. 

If  a  beam  of  solar  rays  be  split  up  by  refraction  in  passing  through  a 

prism,  and  thus  sorted  out  into  a  spectrum  according  to  the  wave-lengths,  the 

rays  of  different  wave-length  can  be  examined   in  turn.     Taking  any  special 

ray.  for  example,  one  whose  length  is  ().()()( )."><)  mm.,  it  will,  if  received  in  the 

•  •ause  the  sensation  of  green  lijjht  :   it  a  thermometer  bulb  is  placed  oefore 


CONTROL.   OF   ATMOSPHERIC    TEMPEBATUBBS    BY    THE    SUN.  25 

it.  a  slight  rise  of  temperature  will  show  that  the  energy  of  the  ray  has  been 
converted  into  heat  ;  certain  specially  prepared  substances  would  be  excited  by 
the  same  ray  to  rearrange  themselves  chemically.  How  impossible  is  it  then 
to  base  a  terminology  suitable  for  the  rays  of  insolation  on  the  effects  that 
they  produce  upon  various  substances. 

While  the  sensory  nerves  of  the  body  can  detect  no  differences  among  rays 
of  different  wave-length,  except  in  so  far  as  they  produce  different  amounts  of 
heat,  the  nerves  of  the  eye  have,  by  means  of  their  "rods  and  cones," 
become  able  to  distinguish  between  the  action  of  rays  of  different  wave-lengths, 
and  thus  to  discriminate  what  we  call  colors. 

33.  Actinometry.      It  follows  as  a  corollary  from  what   has   just   been 
stated  that  if  we  wish  to  gain  a  measure  of  the  energy  brought  to  us  by  the 
rays  of  the  sun,  this  can  best  be  done  by  means  of  an  instrument  in  which 
some  good  absorber,   such  as  carbon,  is  allowed  to  absorb  all  the  incident 
insolation  ;  then  if  the  section  of  the  absorbed  beam  is  measured,  if  the  weight 
of  the  absorber  is  known,  and  the  rise  of  temperature  that  it  suffers  in  a  given 
time  is  determined,  a  measure  of  the  value  of  insolation  can  be  gained  and 
compared  with  other  measures.     Instruments  of  this  kind  are  called  actino- 
meters.     They  have  been  used  by  various  observers,  but  by  none  more  success- 
fully than  by  Professor  S.  P.  Langley,  now  Secretary  of  the  Smithsonian 
Institution.     By  means  of  observations  made  when  the  sun  is  high  and  low  in 
the  sky,  or  from  low-level  stations  and  from  mountain  tops,  allowance  has 
been  fairly  made  for  the  loss  of  insolation  in  the  atmosphere  ;  and  it  is  thus 
found  that  a  solar  ray  of  one  square  centimeter  in  cross  section  will  raise  the 
temperature  of  a  gram  of  water  nearly  three  degrees  centigrade  in  one  minute. 
The  amount  of  heat  or  other  form  of  energy  needed  to  raise  the  temperature 
of  a  gram  of  water  one  centigrade  degree  is  called  a  small  calorie.1 

A  solar  ray  of  one  square  centimeter  in  cross  section  will  therefore  yield  if 
totally  absorbed,  nearly  three  small  calories  in  a  minute.  This  is  called  the 
k'  solar  constant."  The  most  energetic  part  of  the  solar  spectrum  is  included 
within  the  so-called  "  light "  rays,  the  infra-red  and  the  ultra-violet  rays  being 
of  relatively  small  intensity  ;  and  there  is  good  reason  for  thinking  that  our 
eyes  have  learned  to  perceive  the  stronger  rays  by  very  reason  of  their 
strength  having  made  them  the  most  easily  perceptible. 

34.  Radiation  from  the   earth.     All   bodies   emit   radiant   energy  in   a 
greater  or  less  degree.     The  sun  is  the  most  energetic  radiator  that  we  know 
of,  unless  an  exception  be  made  in  favor  of  some  of  the  stars  ;  but  with  those 
we  are  not  here  concerned. 

1  Sometimes  the  unit  of  weight  is  taken  as  a  kilogram  ;  the  amount  of  heat  in  that  case 
i>  railed  the  large  calorie.  The  English  unit  of  heat  is  generally  based  on  the  pound  and  the 
Fahrenheit  degree. 


lM  ELEMENTARY    M  ETEOIM  >L(Hi  V. 

Not  only  the  sun,  but  also  the  air,  the  ocean  and  the  land  emit  radiations. 
As  these  bodies  can  only  be  seen  when  illuminated  by  some  source  of  light, 
we  may  infer  that  the  wave-length  of  their  own  rays  is  greater  than  that  of 
the  visible  rays  ;  and  this  has  been  shown  to  be  the  case  by  Langley,  who 
has  measured  their  dimensions  by  direct  experiment,  and  found  them  to  be 
much  coarser  than  any  discovered  in  the  sun's  rays. 

It  is  by  the  emission  of  these  extremely  coarse  rays  that  a  balance  is  main- 
tained with  the  absorbed  insolation,  and  thus  the  temperature  of  the  earth's 
surface  is  held  at  an  average,  intermediate  value.  In  day-time  and  especially 
on  clear  summer  days,  the  temperature  may  rise  to  a  comparatively  high  degree, 
when  insolation  is  at  its  maximum  value  ;  at  night  and  especially  in  clear 
winter  nights,  the  temperature  may  fall  to  a  very  low  minimum  under  the 
almost  unimpeded  action  of  terrestrial  radiation.  But  these  high  and  low 
temperatures  are  simply  oscillations  about  a  mean  value,  dependent  on  the 
balanced  action  of  absorption  and  emission  of  radiant  energy  by  the  earth. 

Recalling  that  the  air,  the  water  and  the  land  act  differently  on  the  insola- 
tion incident  upon  them,  it  may  now  be  added  that  their  activity  in  the 
emission  of  radiant  energy  corresponds  closely  to  their  activity  in  absorption. 
This  is  the  case  with  all  bodies.  Indeed,  were  it  not  so,  those  whose  power  of 
absorption  exceeded  their  radiating  power  would  become  continuously  hotter 
and  hotter.  With  these  various  preliminary  facts  in  mind',  we  may  next 
undertake  the  explanation  of  the  changes  of  temperature  experienced  by  the 
air,  the  water  and  the  land  during  the  changes  from  day  to  night,  or  from 
summer  to  winter. 

35.  Absorption  and  radiation  by  the  atmosphere.  The  upper  air  does 
not  vary  much  from  its  mean  temperature,  as  far  as  the  few  observations  made 
at  great  altitudes  give  us  information.  This  is  because  it  is  a  poor  absorber, 
and  hence  also  a  poor  radiator.  Insolation  passes  through  it  almost  unim- 
peded during  the  day;  and  at  night  its  temperature  is  slowly  reduced,  because 
so  little  of  its  heat  is  expended  in  exciting  radiation. 

The  case  is  somewhat  different  with  the  lower  air.  particularly  over  the 
land;  for  there  the  numerous  dust  particles  aid  both  diurnal  absorption  and 
nocturnal  radiation,  and  the  land-surface  helps  to  \vann  the  air  by  day  and  to 
cool  it  by  night.  The  dust  particles  act  as  absorbers  during  the  day.  and  thus 
b.-come  centers  of  heat  for  the  surrounding  air;  while  at  night  they  are  effec- 
tive radiators,  and  then  reduce  the  temperature  of  the  air  about  them.  The 
laud-surface  by  day  reflects  a  considerable  share  of  insolation,  and  thus  causes 
it  to  pass  a  second  time  through  the  air;  it  also  emits  radiation  more  actively 
as  it  rises  in  temperature,  and  thus  aids  in  warming  the  air.  At  night  the 
hind  cools  to  a  low  temperature,  and  the  air  near  it  is  cnoh-d  by  radiation  to 
its  cold  surface.  If  the  air  becomes  very  dusty,  as  in  desert  regions,  or 


CONTROL  OF  ATMOSPHERIC  TEMPERATURES  BY  THE  SUN. 


'11 


smoky,  as  in  the  neighborhood  of  forest  fires,  or  cloudy,  as  in  stormy  weather, 
it  may  be  that  the  lower  strata  are  shielded  from  warming  by  day  and  from 
cooling  by  night  by  the  many  particles  above  them.  Air  of  ordinary  clean- 
ness may,  therefore,  be  expected  to  possess  a  greater  and  greater  range  of 
temperature  as  we  descend  towards  the  earth,  an  example  of  diurnal  temper- 
atures for  the  lower  air  in  clear  weather  being  given  in  Fig.  10  a;  but  very 
dusty,  smoky  or  cloudy  air  must '  have  a  level  of  maximum  range  of  tempera- 
ture at  some  height  over  the  earth's  surface,  and  a  less  range  both  above  and 
1  it-low  this  height.  An  illustration  of  the  relatively  constant  temperature  in 
the  lower  air  during  a  spell  of  cloudy  weather  is  given  in  Fig.  10  b. 

A  practical  application  of  this  principle  is  seen  in  the  method  commonly 
adopted  in  protecting  tender  plants  from  freezing  in  early  or  late  frosts  by 
building  smoky  fires  about  them,  and  thus  providing  them  with  a  cover  of 
smoke  during  the  night  (Sect.  187).  In  such  cases  no  frost  will  be  formed 
below  the  smoke,  if  it  is  dense  enough,  while  the  frost  may  be  severe  on  the 
surrounding  unprotected  ground.  It  may  be  expected  that  if  a  series  of  ob- 
servations of  temperature  were  taken  at  different  heights  in  a  dense  layer  of 
smoke,  a  greater  nocturnal  fall  of  temperature  would  be  found  near  the  top 
than  at  the  bottom. 


2000  me  t. 


36.  Vertical  temperature  gradient.  The  general  distribution  and  varia- 
tion of  temperature  in  the  atmosphere  may  be  simply  illustrated  in  a  diagram 
(Fig.  3),  in  which  the  vertical  scale, 
OY.  represents  altitude,  being  divided 
into  thousands  of  feet  (the  altitude 
where  pressures  of  29,  28,  27,  etc., 
inches  occur,  is  marked  according  to 
the  table  of  Sect.  19);  and  the  hori- 
zontal scale,  OX,  represents  tempera- 
ture, on  the  Fahrenheit  scale.1  Many 
observations  on  mountains  and  in 
balloons  have  determined  that  on 
the  average  the  temperature  of  the 
atmosphere  diminishes  1°  Fahr.  for 
every  three  hundred  feet  of  elevation. 
This  relation  is  indicated  for  a  station 
at  sea-level  whose  mean  temperature  is 
65°,  by  the  oblique  line,  AB,  whose  in-  FlG- 

cli  nation  is  such  that  for  every  rise  of  three  hundred  feet  on  the  scale  of 

1  The  scale  of  the  several  diagrams  of  this  kind  is  constant  through  the  book.  Small 
crosses  to  tin-  right  of  the  vertical  scale  mark  spaces  of  500  meters;  similar  crosses  beneath 
the  horizontal  scale  indicate  even  5°  on  the  centigrade  thermometer. 


28  ELEMENTARY    METEOROLOGY. 

altitude,  it  shows  a  decrease  of  one  degree  on  the  scale  of  temperature.  The 
rate  of  vertical  decrease  of  temperature,  thus  expressed  either  as  a  numerical 
ratio  or  by  graphic  method  in  a  line,  is  called  the  vertical  temperature 
gradient. 

From  the  explanation  of  the  preceding  section,  we  may  now  add  to  the 
diagram  a  series  of  horizontal  lines,  representing  the  diurnal  range  of  tem- 
perature in  the  air  at  various  altitudes,  the  middle  points  of  the  lines  lying 
on  the  line  of  the  average  vertical  temperature  gradient.  In  the  upper  air  the 
range  lines  are  short;  in  the  lower  air,  especially  over  the  land,  they  are 
longer.  Dotted  lines,  CD,  EF,  connecting  the  ends  of  the  horizontal  lines, 
serve  to  indicate  the  vertical  decrease  of  temperature  in  the  atmosphere  at 
times  of  highest  and  lowest  diurnal  temperatures;  and  as  CD  and  jE^are  dif- 
ferently curved,  it  follows  that  the  vertical  temperature  gradient  is  a  variable 
quantity.1  Its  values  at  different  times  will  be  later  shown  to  exert  a  marked 
control  on  atmospheric  processes. 

The  direct  rays  of  insolation  arriving  at  sea-level  are  weakened  by  absorp- 
tion and  other  losses  on  their  way  through  the  atmosphere.  In  cloudy 
weather  the  largest  part  of  the  insolation  is  detained  in  the  atmosphere; 
under  the  densest  fogs  of  London,  hardly  any  perceptible  rays  from  the  sun 
reach  the  ground.  In  clear  weather  a  vertical  ray  passing  through  the  least 
thickness  of  air  reaches  sea-level  with  about  three-quarters  of  the  intensity 
that  it  is  estimated  to  possess  outside  of  the  atmosphere;  but  the  loss  thus 
suffered  is  partly  made  up  by  the  arrival  of  indirect  rays  that  come  from  the 
open  sky,  having  been  turned  from  their  initial  paths  by  dust  or  cloud  par- 
ticles. When  the  sun  is  below  the  zenith,  the  loss  of  intensity,  as  the  rays 
pass  obliquely  through  the  atmosphere,  is  much  greater  than  that  suffered  by 
a  vertical  ray;  and  at  sunset  the  sun  may  even  be  observed  by  the  unprotected 
eye.  While  any  small  volume  of  air  detains  only  a  minute  part  of  the  inci- 
dent insolation,  the  entire  thickness  of  the  atmosphere  withholds  a  consider- 
able share  of  insolation  from  the  earth.  We  have  now  to  examine  the  effects 
produced  by  the  rays  that  penetrate  to  the  sea  and  land. 

37-  Absorption  and  radiation  by  the  ocean.  The  surface  layer  of  tin; 
ocean,  with  which  we  are  concerned  in  meteorology,  is  like  the  air  in  bein^ 
slow  to  change  its  temperature,  but  for  different  reasons.  This  is  a  matter  <>f 
so  great  importance  in  determining  the  climate  of  many  places  that  it  must  In- 
examined  with  care. 

Under  the  full  glare  of  even  an  equatorial  sun,  the  temperature  of  the 
water  surface  rises  little,  for  the  following  reasons.  First,  a  considerable 

1  It  is  important,  in  employing  graphic  illustrations  of  this  kind,  that  their  meaning 
should  be  frequently  stated  in  well-chosen  words,  and  that  the  atmospheric  conditions  thus 
represented  irraphically  and  verbally  should  be  clearly  conceived  in  the  mind. 


CONTROL  OF  ATMOSPHERIC  TEMPERATURES  BY  THE  SUN.      29 

share  of  the  incident  insolation  is  reflected  away  from  the  surface  of  the 
ocean;  and  this  portion  has  no  effect  whatever  on  the  temperature  of  the 
water.  As  it  passes  out  through  the  atmosphere,  a  small  share  of  it  is 
absorbed,  and  the  rest  escapes  to  interplanetary  space.  Second,  most  of  that 
which  enters  the  water  is  not  absorbed  by  the  surface  layer,  but  is  transmitted 
to  greater  depths;  only  a  little  is  absorbed  near  the  surface,  or  in  any  given 
layer.  Third,  part  of  the  absorbed  insolation  at  the  surface  is  expended  in 
changing  the  state  of  some  of  the  water  from  liquid  to  vapor  or  gas,  and  this 
part  does  not  cause  any  rise  of  temperature.  This  is  an  extremely  important 
matter,  and  will  be  referred  to  again  and  at  length  in  Chapters  VIII  and  IX, 
when  treating  of  the  moisture  and  the  clouds  of  the  atmosphere.  It  may  be 
now  simply  stated  that  the  insolation  thus  expended  in  changing  the  state  and 
not  in  raising  the  temperature  of  the  water  is  called  the  latent  heat  of  evapora- 
tion; and  that  the  evaporation  of  water  requires  a  large  amount  of  latent 
heat.  Recalling  that  a  unit  of  heat  is  the  amount  of  heat  needed  to  raise  the 
temperature  of  a  pound  of  water  one  degree  Fahrenheit,  it  is  found  that  the 
evaporation  of  a  pound  of  water  requires  about  a  thousand  units  of  heat;  or 
remembering  that  a  large  calorie  is  the  amount  of  heat  needed  to  raise  the 
temperature  of  a  kilogram  of  water  one  degree  centigrade,  about  555 
large  calories  will  be  needed  to  evaporate  a  kilogram  of  water;  the  precise 
amount  varying  with  the  temperature  at  which  evaporation  takes  place.  It  is 
evident,  therefore,  that  the  evaporation  of  water  from  the  ocean's  surface  is 
an  effective  means  of  retarding  its  rise  of  temperature.  Fourth,  the  little 
insolation  that  is  absorbed  by  the  surface  layer  of  the  ocean  causes  only  a 
small  rise  of  temperature;  for  it  is  more  difficult  to  raise  the  temperature  of 
water  than  of  any  other  natural  substance.  This  matter  will  be  referred  to 
again  when  considering  the  case  of  the  surface  of  the  land.  Fifth,  the  water 
of  the  surface  of  the  ocean  is  in  almost  continual  motion;  that  which  is  now 
at  the  surface  is  shortly  afterwards  more  or  less  mixed  with  water  from  a 
greater  or  less  depth ;  and  that  which  is  at  one  time  in  the  torrid  zone  under 
the  stronger  rays  of  the  sun,  is  slowly  carried  away  by  the  currents,  and  its 
place  is  taken  by  other,  unwarmed  water.  The  temperature  of  the  water  at 
any  given  place  is  thus  held  down  to  a  moderate  degree  by  the  continual  re- 
moval of  the  warmed  water  and  its  replacement  by  another  less  warmed  volume. 
XKU-IV  all  these  conditions  operate  as  well  in  preventing  a  rapid  fall  of 
temperature  on  the  ocean's  surface  by  radiation  at  night  as  in  retarding  the 
rise  of  temperature  by  day.  Being  a  transparent  substance,  and  having  a 
fairly  good  reflecting  surface,  the  upper  layer  of  water  is  a  poor  radiator.  It 
is  as  difficult  to  cool  water  as  it  is  to  warm  it;  and  the  little  energy  lost  by 
radiation  can,  therefore,  have  particularly  little  effect  in  lowering  its  temper- 
ature. As  the  surface  layer  becomes  somewhat  cooled,  its  place  may  then  be 
taken  by  less  cooled  water  from  a  depth  below  the  surface. 


30  ELEMENTARY  METEOROLOGY. 

The  under  layers  of  the  ocean  beyond  the  action  of  insolation,  are  even 
more  conservative  in  respect  to  changes  of  temperature,  either  diurnal  or 
annual,  than  the  surface  layers;  and  the  great  mass  of  deep  water  is  hardly 
more  variable  in  temperature  than  the  deeper  layers  of  the  solid  earth.  We 
shall  see  the  strong  effects  of  the  comparatively  uniform  temperatures  of  the 
ocean  when  considering  the  climates  of  different  regions;  those  countries  near 
the  ocean,  and  particularly  to  the  leeward  of  large  oceanic  areas,  possess  an 
equable  temperature  the  year  round;  while  those  far  removed  from  the  oceans 
suffer  from  extreme  ranges  of  temperature,  more  fully  explained  in  a  later 
section. 

38.  Relation  of  diurnal  temperature  range  in  air  and  water.  Over  the 
greater  part  of  the  oceans  the  diurnal  range  of  temperature  in  the  surface 
water  is  hardly  one  degree.  The  range  of  temperature  in  the  open  air  close 
to  the  surface  of  the  sea  is  two  or  three  times  as  much.  As  this  relation  is 
the  opposite  of  that  which  we  shall  rind  obtaining  over  the  lands,  it  needs  a 
brief  explanation.  The  small  amount  of  insolation  absorbed  by  the  lower 
layers  of  the  atmosphere  is  all  applied  effectively  to  raising  its  temperature; 
although  little  is  absorbed,  it  is  so  easy  to  warm  a  layer  of  air  that  the  tem- 
perature is  perceptibly  increased.  The  much  larger  amount  of  insolation 
absorbed  by  the  upper  layer  of  the  ocean  waters  is  applied  to  various  tasks; 
only  a  part  of  it  is  applied  to  the  task  of  warming  the  water;  and  this  task 
is  found  so  difficult  that  the  rise  in  the  temperature  at  the  surface  is  hardly 
noticeable.  As  the  cooling  at  night  is  equivalent  in  amount  to  the  warming 
by  day,  it  follows  that  the  lower  air  over  the  greater  or  oceanic  surface  of  the 
globe  has  a  stronger  diurnal  range  of  temperature  than  that  of  the  surface  on 
which  it  rests;  but  as  we  are  dwellers  on  the  land,  this  is  of  less  importance 
t<»  us  than  the  opposite  relation,  namely,  the  stronger  diurnal  range  of  tem- 
perature in  the  surface  layer  of  the  ground  than  in  the  lower  air,  which  we 
now  have  to  consider. 

39.  Absorption  and  radiation  by  the  land.  The  surface  of  the  land  is  in 
many  ways  contrasted  with  that  of  the  ocean.  It  is  a  comparatively  poor 
reflector  ;  but  little  of  the  insolation  incident  upon  it  is  turned  away  to  empty 
space.  It  is  opaque,  and  all  the  insolation  that  is  absorbed  acts  on  a  relatively 
thin  surface  layer.  It  is  a  solid  substance,  and  hence  cannot  equalize  its 
different  temperatures  by  mixture,  as  happens  in  the  ocean  waters.  It  is  non- 
volatile, and  hence  there  is  no  disappearance  of  insolation  in  the  form  of 
latent  heat.  It  is  easily  warmed  in  comparison  with  water,  or  in  physical 
language,  its  specific  heat  is  low  ;  and  hence  the  absorbed  insolation  produces 
a  large  rise  of  temperature.  For  .-ill  these  reasons,  the  surface  of  the  land 
reaches  a  high  temperature  under  strong  sunshine. 


I 
CONTROL    OF    ATMOSPHERIC    TEMPERATURES    BY   THE    SUN.  31 

Conversely,  it  falls  to  a  low,  temperature  at  night.  It  is  a  good  radiator  ; 
all  the  radiant  energy  emitted  is  supplied  from  a  thin  surface  layer  ;  its 
specific  heat  is  comparatively  low  ;  and  hence,  the  active  emission  of  radiant 
energy  from  a  thin  layer  at  the  surface  results  in  a  rapid  fall  of  temperature. 
Where  the  land  is  moist  the  changes  of  temperature  are  less  than  where  it  is 
dry  or  arid.  Where  it  is  covered  by  vegetation  it  is  shielded  both  by  day  and 
night  and  its  changes  in  temperature  are  greatly  reduced.  Moreover,  in  the 
daytime  there  is  much  evaporation  from  the  leaves  of  plants  ;  and,  further- 
more, there  is  then  a  certain  amount  of  work  done  by  insolation  in  separating 
the  carbonic  acid  that  is  absorbed  by  the  leaves  into  its  constituent  parts,  as 
needed  in  the  growtli  of  the  plant.  Both  of  these  processes  tend  to  lower  the 
temperature  that  would  be  otherwise  reached. 

The  form  of  the  land-surface  exercises  a  marked  control  on  its  changes  of 
temperature  from  day  to  night.  In  a  valley,  free,  unreturned  radiation  to  the 
sky  is  diminished  by  enclosure  between  the  hillsides.  On  a  hill-top  or  mountain 
summit,  where  the  horizon  is  unbroken,  radiation  is  most  effective  in  reducing 
the  temperature  of  the  surface.  This  will  be  more  fully  considered  in  the 
chapter  on  climate. 

40.  Inter-radiation  of  air  and  earth.  We  have  thus  far  examined  the 
changes  taking  place  in  the  different  parts  of  the  earth  separately,  as  if  they 
had  no  effect  on  each  other  ;  but  this  is  not  the  case.  The  earth  and  the 
atmosphere  act  on  each  other  by  radiation  and  by  conduction  :  both  of  these 
processes  are  of  moment. 

Just  as  the  rays  from  the  sun  warm  the  surface  of  the  earth,  so  the  rays 
emitted  from  the  surface  of  the  earth  in  the  daytime  aid  in  raising  the 
temperature  of  the  air.  Terrestrial  rays,  however,  are  weak  compared  to  solar 
rays,  and  they  are  not  actively  absorbed  by  the  atmosphere,  but  pass  out 
through  clear  air  with  about  as  little  loss  as  the  solar  rays  suffered  on  entering 
through  it.  The  strongest  control  of  air  temperatures  by  radiation  from  the 
earth  will  be  in  the  lower  air,  near  the  radiating  surface  ;  over  the  land, 
whose  radiation  is  much  stronger  than  that  of  the  oceans  ;  in  valleys,  where 
the  concave  land  surface  partly  encloses  the  air  that  rests  on  it ;  and  when  the 
air  is  somewhat  dusty,  so  as  to  acquire  more  easily  a  share  of  the  energy  that 
passes  through  it. 

Conversely,  at  night  when  the  land-surface  has  fallen  to  a  low  temperature 
by  the  escape  of  its  radiation  through  the  atmosphere,  it  becomes  colder  than 
the  air  near  it ;  then  the  air  cools  by  moderate  radiation  to  the  cold  ground. 
But  although  these  processes  are  important,  it  must  be  observed  that  the 
Changes  of  temperature  thus  produced  in  the  air  are  slow  and  comparatively 
small,  as  well  as  limited  for  the  greatest  part  to  the  lower  layers  of  the 
atmosphere. 


32  ELEMENTARY   METEOROLOGY. 

It  has  been  supposed  that  the  air  was  a  better  absorber  of  terrestrial 
radiation,  than  of  solar  radiation;  and  thus  the  atmosphere  has  been  compared 
to  a  trap  which  allowed  sunshine  to  enter  easily  to  the  earth's  surface,  but 
prevented  the  free  exit -of  radiation  from  the  earth  ;  water  vapor,  in  particular, 
was  thought  to  be  very  active  in  this  selective  process.  The  general  tempera- 
ture maintained  by  the  atmosphere  has  been  explained  largely  on  these  sup- 
positions, but  recent  observations  throw  grave  doubt  on  both  of  them. 
Clear  air  allows  the  coarse-waved  radiation  from  the  earth  an  easy  outward 
passage.  Water  vapor  is,  like  clear  air,  a  poor  absorber  of  nearly  all  kinds  of 
waves.  It  is  true  that  the  presence  of  excessively  fine  water-particles, 
sufficient  only  to  make  the  air  faintly  hazy,  greatly  diminishes  its  power  of 
transmission,  or  diathermance ;  but  water  vapor,  that  is,  water  in  the  gaseous 
state,  is  found  by  experiment  to  be  as  poor  an  absorber  as  pure  dry  air.  The 
temperature  of  the  air  is,  therefore,  now  explained  as  a  result  of  its  own 
absorption  and  radiation,  largely  aided  by  suspended  dust  and  by  certain 
processes  considered  in  the  following  paragraphs. 

41.  Conduction.  It  is  a  matter  of  common  experience  that  a  bar  of  iron 
heated  at  one  end  becomes  heated  at  the  other  end  also.  This  is  explained  by 
the  spreading  of  the  increased  molecular  agitation  from  the  heated  part  to  tin- 
parts  less  heated.  Heat  is  thus  said  to  flow  from  the  hotter  to  the  cooler 
parts  of  a  body  ;  and  the  passage  of  heat  in  this  way  is  called  conduction.  It 
should  be  noticed  that  in  this  process  we  have  nothing  to  do  with  the  conversion 
of  energy  from  one  form  or  manifestation  to  another,  as  was  the  case  both  in 
the  emission  of  insolation  from  the  sun,  and  in  its  absorption  on  the  earth. 
Conduction  does  not  involve  a  transformation  of  energy,  but  only  a  distribution 
of  energy. 

Various  bodies  are  very  unlike  in  their  ability  to  conduct  heat.  Silver  and 
copper  are  good  conductors  ;  stone  and  water  are  poor  conductors.  From  this 
it  appears  that  we  shall  be  little  concerned  with  the  downward  conduction  of 
heat,from  the  surface  of  the  land  or  water  by  day  and  in  summer,  or  with  the 
upward  conduction  by  night  or  in  winter.  On  the  land  the  ordinary  diurnal 
changes  of  temperature  are  extinguished  at  a  depth  of  a  few  feet,  and  the 
annual  changes  are  reduced  to  a  very  small  value  at  a  depth  of  twenty  or 
thirty  feet.  The  downward  propagation  of  summer  warmth  from  the  surface 
slow  that  it  is  not  felt  at  a  depth  of  twenty-five  feet  until  the  following 
winter  ;  and  at  that  depth  the  annual  range  of  temperature  is  reduced  to 
somewhat  less  than  one  twentieth  of  its  value  at  the  surface.  Kig.  .".a 
illustrates  these  facts,  as  determined  by  observations  at  various  depths  at 
Munich,  Bavaria.  The  time  and  depth  at  which  certain  temperatures  occur 
are  indicated  by  the  curved  lines,  the  values  being  given  in  meters  and  centi- 
grade degrees. 


CONTROL  OF  ATMOSPHERIC  TEMPERATURES  BY  THE  SUN. 


33 


is  an  extremely  poor  conductor  of  heat.  The  surface  of  a  sheet  of 
snow  may  become  extremely  cold  during  the  long  clear  nights  of  winter,  when 
radiation  goes  on  almost  unimpeded,  but  this  surface  cooling  is  very  slowly  pro- 
pagated downward,  and  its  amount  rapidly  decreases  underground. 

The  air  as  a  whole  varies  in  temperature  so  slowly  from  part  to  part  that 
conduction  within  its  mass  has  little  play;  except  in  the  minute  way  of 
gaining  heat  from  dust  particles  by  day,  and  losing  it  to  them  by  night,  as  has 


4m 

5m 


\ 


V 


^^ 


\ 


FIG.  3a. 

already  been  referred  to.  But  with  the  lower  strata  of  air,  in  contact  with 
the  sea  and  the  land,  the  case  is  different.  Here  it  often  happens  that  a 
considerable  difference  of  temperature  exists  between  the  air  and  the  surface 
on  which  it  lies,  even  within  a  distance  of  a  few  feet ;  and  at  such  times, 
conduction  is  effective.  It  is  aided  by  radiation,  for  this,  like  conduction, 
varies  directly  with  the  contrast  of  temperature,  and  inversely  with  the 
distance  between  the  bodies  concerned :  but  for  the  moment,  let  us  consider 
chiefly  the  case  of  conduction. 

42.  Conduction  of  heat  between  the  air  and  the  land.  Consider  the  case 
of  a  high  plateau  in  a  northern  latitude  during  a  long  winter  night.  Let  ij  be 
far  from  the  tempering  effects  of  any  ocean,  as  in  the  center  of  the  great  land 
area  of  Europe-Asia.  The  surface  of  the  barren  ground  must  become  exces- 
sively cold.  The  thin,  pure  air  above  its  elevated  surface  offers  slight  impedi- 
ment to  the  escape  of  its  heat  by  radiation  ;  the  dryness  of  such  a  region 
ensures  that  the  sky  shall  be  cloudless  and  that  little  or  no  vapor  shall  be 
condensed  on  the  ground  to  retard  the  cooling  by  the  liberation  of  latent  heat ; 
the  sunshine  of  daytime  is  weak  and  lasts  only  a  few  hours,  thus  allowing  the 
process  of  cooling  to  go  on  night  after  night  with  small  interruption.  While 
the  ground  thus  cools  rapidly  to  a  low  temperature,  the  thin,  clean  air  high 
above  it  cools  but  little  ;  but  the  layer  of  air  next  to  the  surface  of  the  plateau, 
being  in  the  neighborhood  of  a  much  colder  body,  loses  much  of  its  heat  by 


34 


ELEMENTARY    METEOROLOGY. 


Y 

-4-5000  met. 
-16000' 

-16000' 


conduction  to  the  cold  ground ;  for  while  the  air  cannot  carry  much  heat  by 
conduction,  the  little  heat  that  it  does  carry  suffices  very  effectively  to  reduce 
the  temperature  of  a  substance  so  light  and  of  so  low  a  specific  heat  as  air  is. 
Supplemented  by  radiation,  the  actual  cooling  of  the  air  near  the  ground  at 
such  a  time  is  much  greater  than  that  of  the  air  above  it. 

/\43.  Inversions  of  temperature.  Reference  has  already  been  made  to  the 
general  decrease  of  temperature  encountered  as  we  ascend  in  the  atmosphere  ; 
but  in  the  case  of  the  air  over  a  dry  plateau  on  a  long  winter  night,  the  cooling 
of  the  lower  layers  may  be  so  great  as  to  reduce  them  to  a  decidedly  lower 
temperature  than  that  of  the  air  at  the  height  of  several  hundred  feet  aloft. 
Such  a  condition  is  known  as  an  inversion  of  temperature.  It  may  be  illustrated 
in  the  following  diagram. 

Recalling  the  explanation  given  in  Section  36,  we  have  in  Fig.  4  the  mean 
vertical  temperature  gradient,  AB,  indicating  the  usual  rate  of  decrease  of 

temperature  upwards  from  the  plateau  surface. 
This  condition  may  prevail  about  sunset,  the  tem- 
perature of  the  air  then  being  between  the  extremes 
of  high  noon  and  late  night.  When  the  sun's  rays 
are  no  longer  felt,  the  cooling  that  had  begun  in 
the  afternoon  is  continued  for  a  time  more  rapidly  ; 
and  the  whole  mass  of  the  atmosphere  is  somewhat 
reduced  in  temperature,  as  indicated  by  the  hori- 
zontal lines  at  various  altitudes.  At  the  same 
time,  the  surface  of  the  ground  cools  much  more 
rapidly,  and  by  midnight  it  may  have  fallen  to  a 
temperature  close  to  Fahrenheit  zero.  The  air 
near  it  is  also  greatly  cooled  by  radiation  and 
conduction  to  its  cold  surface,  and  before  morning 
falls  to  a  temperature,  K,  much  lower  than  that  of 
the  air  at  6",  a  thousand  feet  above  the  ground. 
The  decrease  of  temperature  by  radiation  from  the 
ground  progresses  rapidly  at  first,  when  it  is  but 
little  cooler  than  the  air  above  it ;  but  late  at  night,  when  a  strong  contrast  of 
temperature  between  ground  and  air  is  developed,  further  cooling  of  the 
ground,  and  thus  of  the  air  close  to  it,  is  somewhat  checked  by  radiation  from 
the  warmer  air  about  the  height  of  G.  The  strong  curvature  of  the  line  AY/'/-', 
representing  the  peculiarly  reversed  vertical  temperature  gradient  in  the  lower 
air  at  the  late  hour  of  greatest  cold,  gives  clear  illustration  of  the  conditions 
attending  such  inversions  of  temperature  as  an-  licit-  considered. 

As  the  lower  air  cools,  its  expansive  force  d.-civascs  ;   the  overlying  air,  no 
longer  borne  up  by  expansive  force  equal  to  its  weight,  settles  down  a  small 


t' 

6\A    ,  X 


FIG,  I. 


CONTROL    OF    ATMOSPHERIC    TEMPER  A I Tit  ES    BY    THE    SUX.  35 

distance,  compressing  the  air  beneath,  and  thus  increasing  its  density  and 
restoring  its  expansive  force  to  its  former  equality  with  the  weight  from 
above.  This  process  is  not  intermittent  in  nature,  but  is  continually  operating 
at  every  level  in  the  atmosphere  to  maintain  the  equality  between  the  down- 
ward weight  from  above  and  the  upward  expansive  force  from  below. 

Inversions  of  temperature  are  of  much  commoner  occurrence  than  is  gen- 
erally understood.  They  probably  occur  to  a  greater  or  less  degree  every  clear 
night  on  our  dry  western  plains.  Examples  of  their  effects  may  often  be  seen 
in  a  small  way  in  late  spring  frosts,  when  the  lower  leaves  of  a  shrub  may  be 
nipped,  while  the  upper  branches  are  unharmed.  In  a  larger  way,  and  aided 
by  other  processes,  the  milder  temperature  of  low  hills  than  of  adjacent  val- 
ley bottoms  at  night  will  be  explained  in  Section  249.  It  will  also  be  shown 
in  Section  159  that  the  quietness  of  the  air  at  night  depends  largely  on  the 
occurrence  of  or  approach  to  temperature  inversions  of  the  kind  thus  explained. 

Other  examples  of  conduction  might  be  mentioned  in  the  case  of  winds  of 
one  temperature  blowing  over  land  or  water  of  another;  but  as  this  involves 
the  movement  of  the  air  in  large  currents,  it  will  be  postponed  to  Section  193. 

44.  Convection  in  water.  There  is  another  process,  called  convection,  by 
which  unlike  temperatures  are  partially  equalized  in  liquids  or  gases.  This  is 
of  great  importance  in  the  atmosphere.  It  may  be  first  illustrated  by  a  simple 
example  in  the  case  of  water. 

When  a  vessel  of  water  is  heated  at  the  bottom,  the  warmed  layer  is  ex- 
panded and  thus  made  lighter  than  an.  equal  volume  of  cooler  water  above  it. 
In  consequence  of  this  unsteady  arrangement,  the  heavier  overlying  water  is 
drawn  downward  by  gravity,  displacing  the  bottom  layer,  which  then  rises  to 
the  surface.  It  is  our  common  habit  simply  to  say  that  the  warmed  lighter 
layer  ascends;  but  it  must  not  be  forgotten  that  its  rise  is  a  passive  process, 
and  that  the  really  active  process  is  the  descent  of  the  overlying  water,  which 
is  drawn  down  by  gravity.  By  coloring  the  bottom  layer,  its  ascent  through 
the  overlying  layer  may  be  easily  perceived.  If  the  temperature  be  at  first  uni- 
form throughout,  it  will  be  noticed  that  the  warmed  water  from  the  bottom  is  A 
raised  to  the  very  top  of  the  liquid,  maintaining  its  higher  temperature  all  the.n 
way,  except  for  a" slight  loss  by  conduction  and  mixture  during  ascent;  while 
all  the  rest  of  the  water  settles  down  a  little  distance  towards  the  bottom. 
Then  the  new  bottom  layer  repeats  the  process;  and  so  a  circulatory  motion  is 
established.  This  is  called  a  convectional  circulation,  and  by  its  means  the 
entire  volume  of  water  will  be  warmed  to  almost  as  high  a  temperature  as  is 
maintained  at  the  bottom.  It  depends  essentially  on  the  disturbance  of  a  con- 
dition of  rest  by  the  introduction  of  a  change  in  the  temperature  and  a  conse- 
quent change  in  the  density  of  the  water,  which  is,  therefore,  followed  by 
motion  under  the  action  of  gravity.  After  this  deliberate  explanation  of  the 


36  ELEMENTARY    METEOROLOGY. 

convectional  process,  its  further  statement  may  be  made  more  brief  by  speak- 
ing only  of  the  ascent  of  the  warm  under  layer,  with  which  we  are  generally 
most  concerned. 

X45.  Conduction  and  convection  in  the  atmosphere.  Conduction  in  the 
atmosphere  was  illustrated  by  the  cooling  of  the  lower  air  at  night,  when  it 
lost  heat  chiefly  to  the  colder  surface  of  the  ground  beneath.  This  change  of 
temperature  is  not  followed  by  convection,  for  it  leaves  the  heaviest  layer  of 
air  at  the  bottom,  and  does  not  give  gravity  any  opportunity  to  cause  motion. 
In  the  day-time,  however,  conduction  is  followed  by  convection,  which  then 
becomes  an  active  process.  Let  us  consider  the  case  of  the  air  over  a  dry 
plain,  beneath  an  unclouded  torrid  sun.  The  ground  warms  rapidly  in  the 
morning,  and  soon  becomes  hotter  than  the  air  which  rests  upon  it.  Conduc- 
tion, aided  as  at  night  by  radiation,  increases  the  temperature  of  the  surface 
stratum  of  air.  This  stratum  then  expands,  and  lifts  up  the  overlying  air  by 
a  small  amount,  thus  reversing  the  process  of  the  night  before.  A  peculiar 
optical  effect  may  then  be  produced,  which  must  be  considered  briefly. 

46.  Mirage.1     As  the  morning  advances,  the  lower  layer  of  air  on  a  level 
surface  may  become  so  superheated,  while  still  lying  for  a  time  beneath  the 
cooler  heavier  air,  as  to  gain  a  strong  vertical  temperature  gradient  near  the 
ground  and  produce  the  singular  effect  known  as  mirage.     This  is  seen  when 
the  eye   of   the  observer  is  a  little   above   the  surface  of   the  superheated 
stratum,  so  as  to  receive  the  rays  of  light  that  have  been  reflected  from  it; 
thus  frequently  causing  it  to  be  mistaken  for  a  sheet  of  water,  with  whose 
reflection  of  oblique  rays  from  the  sky  to  the  eye  we  are  familiar.     Mirages 
of  this  kind  are  often  observed  on  our  barren  western  plains  (Sect.  72). 

A  perfectly  stagnant  atmosphere  might  be  imagined  in  which  the  alternate 
cooling  by  night  and  warming  by  day  caused  a  corresponding  rise  and  fall  of 
the  upper  atmosphere  once  in  twenty-four  hours.  In  this  case  the  work  done 
in  lifting  up  the  upper  air  by  day  would  be  equal  to  that  done  in  compressing 
the  lower  air  at  night.  But  such  a  process  can  hardly  be  supposed  to  proceed 
without  interruption  by  currents  of  air  of  some  kind. 

47.  Dust  whirlwinds.     It  is  not  uncommon  for  desert  mirages  to  dis- 
appear rather  suddenly,  and  at  the  same  time  a  local  dust  whirlwind  springs 
up.     This  means  that  the  superheated  lower  layer  that  has  lain  for  a  time 
delicately  balanced  under  the  heavier  overlying  air,  like  a  layer  of  oil  under  a 
sheet  of  water,  at  last  loses  its  balance  and  literally  upsets.     It  then  drains 
away  upward,  being  urged  to  ascend  by  the  descent  of  the  heavier  overlying 
air.     The  whirling  of  the  ascending  current  results  only  because  all  the  linos 
of  indraft  towards  the  point  of  upward  escnpc   fail  to  meet  precisely  at  the 

1  A  word  of  French  origin,  meaning  reflection;  pronounced  meerazh. 


CONTROL    OF    ATMOSPHERIC    TEMPERATURES    BY    THE    SUN.  37 

center;  they  miss  their  aim  to  one  side  or  another,  and  thus  establish  a  rotary 
motion,  which  once  assumed  is  not  easily  stopped.  As  the  motion  becomes 
brisk,  dust  particles  are  gathered  up  by  it,  vibrations  are  excited  in  its  spiral 
currents,  and  the  whirlwind  becomes  visible  and  audible.  The  dusty  columns 
thus  produced  may  rise  to  a  height  of  a  thousand  or  more  feet,  where  the  air 
.•Hi-rents  spread  out  horizontally.  Such  whirls  are  not  common  on  uneven 
surfaces,  for  there  the  lower  air  does  not  remain  long  enough  close  to  the 
ground  to  become  superheated;  nor  are  they  seen  frequently  on  surfaces  cov- 
ered with  vegetation,  even  though  level;  partly  because  such  surfaces  are 
seldom  so  hot  as  desert  surfaces;  partly  because  less  dust  lies  upon  them,  by 
which  the  ascending  whirls  might  be  made  visible.  But  the  convectional 
ascent  of  the  surface  air  in  a  small  way  is  easily  perceived  on  almost  any 
warm,  clear,  quiet  day  by  looking  over  the  brow  of  a  gentle  rise  in  the  ground; 
the  air  is  then  seen  to  be  "unsteady."  an  appearance  due  to  the  passage  close 
past  one  another  of  small  currents  and  films  of  air  of  different  temperatures, 
in  which  the  rays  of  light  are  irregularly  refracted.  The  same  appearance 
may  be  seen  close  along  side  of  a  hot  stove,  and  for  the  same  reason. 

It  is  manifest  that  convection  must  have  much  influence  in  raising  the 
temperature  of  the  air  during  the  day-time;  for,  as  long  as  it  continues,  one 
layer  after  another  is  brought  close  to  the  ground,  where  it  is  most  effectively 
wanned,  and  whence  it  ascends  to  considerable  altitudes  in  the  atmosphere. 
Moreover,  if  no  convection  took  place,  the  land-surface  and  the  air  lying  close  to 
it  would  become  unsupportably  hot  under  strong  sunshine.  In  warm  seasons 
and  regions  the  convectional  ascent  of  the  lower  air  may  reach  a  height  of 
several  miles  during  the  hotter  hours  of  the  day,  while  at  night  the  effective 
cooling  of  the  air  by  conduction  and  radiation  to  the  ground  is  limited  to  a 
layer  a  few  hundred  feet  thick. 

^   48.     Difference  between  convection  in  liquids  and  in  gases.     The  convec- 
tional circulation  of  liquids  does  not  involve  any  change  of  temperature  in  the 
ascending  and  descending  currents,  except  such  as  may  follow  mixture  and 
conduction.     With  gasgs  an  important  change  of  temperature  occurs;  a  cool-  f- 
ing  in  the  ascending  currents,  and  a  warming  in  the  descending  currents  of  the  y 
circulation.     This  is  entirely  independent  of  the  action  of  mixture  and  con/ 
duction.     It  may  be  briefly  explained  as  follows. 

/x49.  Change  of  temperature  in  vertical  currents.  The  lower  air,  about  to 
ascend,  has  a  certain  temperature  and  a  corresponding  expansive  force  when  it 
begins  to  rise.  As  it  reaches  higher  levels,  the  pressure  upon  it  is  less;  it 
therefore  expands,  pushing  away  the  surrounding  air  to  make  room  for  itself, 
until,  as  a  result  of  its  expansion,  its  expansive  force  is  reduced  to  equality 
with  the  pressure  upon  it.  It  follows,  however,  from  experiment,  as  well  as 
from  the  mechanical  theory  of  heat,  that  in  pushing  away  the  surrounding  air, 


38 


ELEMENTARY    METEOROLOGY. 


i 


the  ascending  air  must  expend  some  of  its  energy;  and  this  expenditure  is 
drawn  from  its  store  of  energy  in  the  form  of  heat;  hence  the  ascending  air 
s  cooled  by  the  very  processes  involved  in  its  ascent.  The  rale  of  cooling 
thus  produced  is  accurately  known;  being  1.6°  on  the  Fahrenheit  scale  for 
every  three  hundred  feet,  or  1°  on  the  centigrade  scale  for  100  meters  of 
ascent.  A  similar  change,  but  of  the  reverse  order,  occurs  in  the  descending 
members  of  the  convectional  circulation.  As  the  Descending  air  settles  down, 
other  air  rolls  on  top  of  it;  it  is  thereby  compressed  to  a  slightly  greater 
density,  and  its  temperature  is  raised.  When  air  is  thus  changed  in  tempera- 
ture, it  is  said  to  be  mechanically  warmed  or  cooled.  Such  changes  are  also 
called  adiabatic,  meaning  thereby  that  they  are  produced  without  the  passage 
of  heat  to  or  from  the  air. 


/  50.  Conditions  of  local  convection  in  the  atmosphere.  The  general 
account  of  convection  now  given  makes  it  clear  that  this  process  cannot 
take  place  at  night,  when  the  air  011  the  ground  is  colder  and  consequently 
heavier  than  that  above  it;  on  the  other  hand,  convectional  overturning  must 
occur  in  the  day-time,  for  then  the  .  bottom  air  is  warmed,  and  may  thus 
become  light  compared  to  that  above  it.  But  the  precise  amount  of  tempera- 
ture contrast  between  the  surface  layers  and  the  overlying  air,  or  in  other 
words,  the  precise  value  of  the  vertical  temperature  gradient  that  will  allow 
convection,  remains  to  be  determined.  A  closer  understanding  of  this  prob- 
lem may  be  gained  from  the  following  diagrams. 

r^  51.  Nocturnal  stability.  Let  EKF,  Fig.  5,  represent  the  value  of  the 
vertical  temperature  gradient  in  the  quiet  nocturnal  air  over  a  plain  at  a  time 

of  temperature  inversion.  Suppose  a 
small  volume  of  the  surface  air  is  raised 
to  the  altitude,  H.  As  it  ascends,  its  tem- 
perature will  decrease  at  the  adiabatic 
rate  of  1.6°  for  every  three  hundred  fret 
of  ascent.  This  rate  is  constant  at  what- 
ever temperature  the  ascent  begins;  it  is 
indicated  by  the  inclined  line,  or  adiabatic 
gradient,  EG.1  When  the  surface  air  has 
risen  to  the  height,  //,  its  temperature 
will  be  lowered  to  L,  its  altitude  and  tem- 
perature being  both  indicated  by  the  point  , 
G.  The  temperature  of  the  surrounding 
air  at  tin-  height,  //.  is  A'';  lienee  tin-  ail- 
that  has  been  raised  has  a  temperature  A"7,  «le^rees  lower  than  that  of  the  ;iir 

1  In  all  diagrams  of  this  kind  the  adiabatic  rate  of  cooling  will  be  indicated  by  straiirht 
broken  lines. 


r 


CONTROL    OF    ATMOSPHERIC    TK.MI'KIJATl'lIKS    BY    THE    SUN. 


39 


that  it  has  risen  into.  It  will  therefore  be  much  heavier  than  the  surrounding 
air,  and  consequently,  if  no  longer  sustained,  it  will  sink  down  to  the  ground 
before  finding  any  air  of  its  own  temperature.  It  must  be  concluded  from 
this  that  the  lower  air  on  plains  during  clear,  quiet  nights  is  not  disposed  to 
move;  and  that  if  disturbed,  it  will  tend  to  return  to  the  position  that  it  had 
before  the  disturbance.  The  air  is  then  said  to  be  in  a  stable  equilibrium. 


2000- 


.  Diurnal  instability.  Consider  next  the  conditions  found  at  noon, 
when  the  lower  air  has  been  warmed  many  degrees,  and  the  vertical  tempera- 
ture gradient  has  taken  the  value, 
C3ID,  Fig.  6.  Kepeat  the  imaginary 
experiment  of  raising  a  small  mass  of 
surface  air  to  a  height,  JV.  From 
having  a  temperature,  C,  at  the  ground, 
it  will  be  mechanically  cooled  by  ex- 
pansion to  a  temperature  correspond- 
ing to  the  point  N.  The  surrounding  ±000 
air  at  the  same  height  has  a  tempera- 
ture, J/,  or  MN  degrees  cooler  than 
that  of  the  air  that  has  been  raised 
from  the  ground.  The  latter  will 
therefore  be  lighter  than  the  air  into 
which  it  has  risen,  and  it  will  con- 
tinue to  ascend,  cooling  at  the  adia- 
batic  rate  as  it  goes  (no  account  being 
taken  for  the  present  of  loss  of  heat 
by  radiation  or  conduction),  until  it 
encounters  air  of  its  own  temperature,  as  at  Z>,  where  it  will  spread  out  later- 
ally. Above  this  level  it  cannot  rise,  for  at  greater  heights  it  would  become 
colder  than  the  surrounding  air.  The  excess  of  the  temperature  at  C  over 
that  at  M,  Fig.  6,  is  greater  than  occurs  in  nature. 

The  value  of  the  vertical  temperature  gradient  at  the  time  when  the 
stability  of  night  was  changed  to  the  instability  of  day  should  be  determined. 
The  change  must  have  made  its  appearance  near  the  ground,  where  the  warm- 
ing of  the  air  proceeds  most  rapidly  in  the  early  morning.  Instability  occurs 
as  soon  as  the  line  of  the  vertical  temperature  gradient  is  carried,  in  shifting 
from  its  nocturnal  to  its  diurnal  form,  past  parallelism  with  the  adiabatic  line. 
From  this  time  on,  till  the  warmest  hour  of  the  day  is  reached,  the  tempera- 
ture of  the  middle  air  will  depend  chiefly  on  the  convectional  ascent  of  air 
that  has  been  warmed  close  to  the  surface  of  the  ground. 

The  same  diagram  may  be  used  to  determine  the  altitude  at  which  the  con- 
vectional ascent  of  the  lower  air  will  be  most  rapid.  This  will  be  where  the 


30  ^ 


.4°+  50. 


FIG.  6. 


40  ELEMENTARY    METEOROLOGY. 

temperature  of  the  ascending  air  exceeds  by  the  greatest  amount  the  tempera, 
ture  of  the  air  through  which  it  ascends;  or  at  the  height,  J/,  where  the 
gradient  line  is  parallel  to  the  adiabatic  line.  Moreover,  all  the  air  below  this 
altitude  is  unstable,  compared  to  the  air  for  a  certain  distance  above  M.  The 
instability  of  the  surface  air  is  stronger  than  that  of  any  layer  above  it;  the 
surface  air  will  ascend  to  a  greater  height  in  the  atmosphere.  The  air  at  a 
height,  P.  may  ascend  to  the  height,  Q;  but  the  air  from  the  ground  may 
ascend  to  D.  It  is  manifest,  however,  that  unless  the  ascending  mass  is  of 
greater  volume  than  would  ordinarily  be  found  in  diurnal  convection,  its  tem- 
perature would  be  reduced  by  mixture  and  conduction,  as  well  as  by  expansion, 
during  ascent;  and  hence  it  would  find  air  of  its  own  temperature  and  cease 
rising  at  some  level,  If,  of  less  altitude  than  Z>.  At  the  same  time,  all  the 
air  through  which  it  rises  would  be  warmed  by  the  heat  taken  from  the  ascend- 
ing current.  Thus  convection  is  effective  in  warming  the  lower  atmosphere. 
The  stronger  the  excess  of  temperature  in  the  lower  air,  the  higher  it  may 
ascend,  and  the  more  effective  it  will  be  in  warming  the  air.  Convection  will 
therefore  characterize  the  day-time  of  warm  seasons  and  hot  regions  of  the 
land,  and  in  those  seasons  and  regions,  a  considerable  thickness  of  the  lower 
atmosphere  will  be  warmed  by  this  process. 

53.  Explanation  of  convection  by  analogy.     In  any  such  process  as  this, 
in  which  motion  follows  a  change  of  temperature,  we  find  an  interesting  illus- 
tration of  the  expenditure  of  solar  energy  in  the  performance  of  work  on  the 
earth.     It  may  be  compared  with  the  running  of  a  clock  by  a  weight.     We 
wind  up  the  weight  against  gravity  by  the  expenditure  of  muscular  energy, 
which  is  only  solar  energy  conveniently  stored  for  use  when  wanted.     Gravity 
then  pulls  down  the  weight  and  sets  in   motion  a  train  of  wheels  whose 
velocity  is  determined  by  the  resistance  of  'the  escapement  under  control  of 
the  pendulum. 

In  an  analogous  manner  the  sun  warms  the  lower  air,  which  expands  and 
raises  the  upper  air  against  gravity;  gravity  then  pulls  down  the  upper  air, 
and  in  so  doing  it  sets  certain  currents  in  motion  at  a  velocity  determined  by 
the  resistances  they  encounter.  Whether  the  motion  is  that  of  a  great  whirl, 
wind  or  of  a  little  filament  of  ascending  air,  it  is  in  all  cases  the  result  of  the 
descent  somewhere  else  of  a  mass  of  air  that  lias  been  raised  against  gravity 
by  the  action  of  insolation. 

54.  Local  convection  illustrated  by  clouds.     A  familiar  effect  of  convec- 
tion  and  of  the  adiabatic  decrease  of  temperature  that  goes  with  it  maybe 
M-i-n  on  nearly  every  fair  summer  day  in  the  formation  of  clouds  of  greater 
and  greater  size  as  noon  approaches,  all  with  rather  even  bases  at  about  the 
same  altitude  of  a  few  thousand  feet,  and  with  rounded  summits  which  may 


CONTROL  OF  ATMOSPHERIC  TEMPERATURES  BY  THE  SUN.      41 

often  be  seen  to  grow  upwards,  if  watched  riwhilly.  Such  clouds  result  from 
the  convectional  ascent  of  the  lower  air  under  the  action  of  sunshine ;  for  as 
the  air  rises,  it  gradually  cools  until  its  vapor  begins  to  condense  ;  then  clouds 
begin  to  form ;  and  as  in  a  given  region  the  lower  air  has  a  tolerably  uniform 
temperature  and  moisture  near  the  ground,  the  base  of  all  the  clouds  formed 
in  this  way  on  any  morning  will  be  at  about  the  same  height.  Condensation 
thus  begun  continues  as  long  as  the  upward  current  is  maintained  ;  the  convex 
form  of  the  top  of  the  ascending  current  is  clearly  shown  in  the  rounded 
form  of  the  cloud  that  is  produced  in  it.  Clouds  of  this  kind,  known 
as  cumulus  clouds,  are  common  in  fair  summer  weather  over  our  Atlantic 
states  ;  they  are  not  so  common  further  in  the  interior  because  there  the 
surface  air  is  drier,  and  a  higher  convectional  ascent  is  necessary  to  produce 
them.  They  are  rare  on  deserts,  for  in  spite  of  the  active  convection  of  such 
regions,  the  surface  air  is  so  dry  that  ascending  currents  are  not  cooled 
enough  by  the  expansion  of  their  ascent  to  make  them  cloudy.  (See  also 
Sections  196-201.) 

When  the  dust  whirls  of  desert  plains  are  carefully  watched  they  may  be 
seen  to  spread  out  laterally  after  reaching  a  certain  height.  This  means  that 
at  that  height  their  ascending  air  is  cooled,  chiefly  by  expansion,  to  the  same 
temperature  as  that  of  the  air  into  which  it  has  risen  ;  above  that  height  it 
cannot  go.  In  thunder-clouds  also,  which  are  simply  examples  of  convection 
on  a  larger  scale,  a  height  is  reached  at  which  the  temperature  of  the  ascending 
current  is  reduced  to  equality  with  that  of  the  air  into  which  it  ascends  ;  at 
that  level  the  cloud-bearing  current  spreads  out  laterally  and  produces  the  flat 
outspreading  cloud-cover  by  which  thunder-clouds  may  be  recognized  from 
afar,  even  when  their  thunder  cannot  be  heard,  and  when  their  bases  are  below 
the  horizon.  This  will  be  more  fully  considered  in  the  chapters  on  clouds  and 
local  storms. 

It  follows  from  the  preceding  paragraphs  that  our  atmosphere  cannot  have 
a  uniform  vertical  distribution  of  temperature  as  long  as  convectional  motions 
take  place  in  it.  However  active  the  convection,  however  warm  the  lower  air, 
it  must  cool  as  it  rises.  However  long  the  process  is  continued,  the  upper  air 
can  never  become  as  warm  as  the  lower  air. 

55.  General  vertical  distribution  of  temperature.  The  foregoing  deliberate 
examination  of  the  processes  of  absorption,  radiation,  conduction  and  convection 
should  enable  the  reader  to  understand  clearly  the  general  vertical  distribution 
of  temperature  in  the  atmosphere. 

The  upper  air,  pure  and  dry,  free  from  clouds  and  dust,  far  from  the 
surface  of  the  earth  and  out  of  reach  of  ordinary  convectional  action,  must 
possess  a  low  temperature  and  must  change  its  temperature  slowly  and  by 
small  amounts. 


42  ELEMKNTAKV    MKTK<  UIOlAHJ  Y. 

The  lower  air,  containing  many  dusty  impurities  and  sustaining  numerous 
clouds,  lying  near  the  surface  of  the  sea  or  land,  must  generally  possess  higher 
temperatures  than  the  upper  air  and  must  generally  agree  closely  with  the 
temperature  of  the  surface  on  which  it  rests.  If  011  the  ocean,  its  diurnal 
variations  of  temperature  are  small,  even  though  a  little  greater  than  those  of 
the  ocean's  surface  ;  the  temperature  of  the  air  at  sea  will  vary  chiefly  with 
changes  of  the  wind.  If  on  the  land,  the  temperature  of  the  air  varies  over  a 
strong  diurnal  range,  and  the  variation  thus  produced  is  greater  than  that 
ordinarily  caused  by  changes  of  the  wind  over  a  large  part  of  the  torrid  land 
area.  In  the  temperate  zone  the  diurnal  changes  are  strongest  in  the  summer 
season  and  in  clear  weather,  but  in  winter  they  are  exceeded  by  the  warming 
or  cooling  that  accompanies  the  stormy  shifts  of  the  wind,  as  will  be  explained 
in  the  chapter  on  storms  and  further  considered  in  the  account  of  the  weather. 

56.  Review.  We  are  now  prepared  to  appreciate  the  actual  distribution 
of  temperature  over  the  earth  in  time  and  place.  The  arrangement  of  the 
atmosphere  about  the  earth  has  been  examined.  The  physical  processes 
involved  in  the  control  of  atmospheric  temperatures  by  the  sun  have  been 
carefully  studied.  The  terrestrial  sphere  may  be  conceived  as  turning  rapidly 
on  its  axis  as  it  moves  along  its  orbit,  always  exposing  a  half  of  its  surface  to 
the  sun  and  thus  intercepting  the  minutest  portion  of  the  vast  shower  of 
radiant  energy  emitted  by  that  enormous  globe.  With  the  changes  from 
day  to  night  and  from  winter  to  summer  every  part  of  the  earth  is  shone 
upon.  While  the  parts  in  shadow  are  cooling,  those  under  sunshine  are 
warming;  and  the  increase  of  temperature,  gained  chiefly  at  the  bottom  of 
the  atmosphere,  has  been  found  to  excite  vertical  interchanging  currents  by 
which  a  considerable  thickness  of  air  is  warmed.  The  next  chapter  might 
naturally  be  concerned  with  the  temperatures  at  different  parts  of  the  world 
and  in  different  seasons  of  the  year ;  but  this  will  be  postponed  until 
another  effect  of  insolation  is  examined. 


THE   COLOKS    OF    THE   SKY.  43 

\ 

CHAPTER   IV. 

THE    COLORS    OF    THE    SKY. 

57.  The  facts  to  be  explained.  The  colors  of  the  atmosphere  include 
those  of  the  open  sky,  of  the  hazy  air,  and  of  the  suspended  clouds.  The 
colors  of  clouds  will  be  considered  in  a  later  chapter  in  connection  with  the 
clouds  themselves.  The  colors  of  the  open  sky  are  here  briefly  described  and 
explained.  It  is  advisable  to  consider  them  under  two  conditions  of  illumina- 
tion :  first,  when  the  sun  stands  at  a  considerable  height  above  the  horizon ; 
second,  when  the  sun  is  near  rising  or  setting,  either  above  or  below  .the 
horizon. 

Daytime  colors.  The  colors  of  the  clear  sky  when  the  sun  is  ten  or  more 
degrees  above  the  horizon  are  for  the  most  part  shades  of  purer  or  paler  blue, 
becoming  white  and  glaring  in  the  close  neighborhood  of  the  sun  and  turning 
pale  or  whitish  towards  the  horizon.  The  clearer  the  weather,  the  purer  the 
blue,  and  the  less  the  share  of  white  both  near  the  sun  and  upon  the  horizon. 
The  higher  the  observer  rises  above  sea-level  on  mountain  peaks  or  in  balloons, 
the  deeper  the  blue  ;  the  illumination  of  the  sky  is  indeed  then  fainter,  but 
the  color  produced  is  stronger.  In  the  lower  air  the  blue  color  fades  away  as 
haze  increases,  and  the  sky  becomes  whitish  and  more  luminous  ;  it  may  turn 
dull  gray  or  yellowish  when  suspended  matter  is  in  great  abundance,  as  in  the 
neighborhood  of  forest  fires. 

Sunset  and  sunrise  colors.  When  the  sun  approaches  the  horizon  and 
passes  below  it,  the  intensity  of  sky-light  decreases  and  the  variety  of  color 
increases  very  greatly.  As  the  sun  sinks  out  of  sight  the  most  marked  change 
from  the  blue  of  the  open  sky  is  seen  in  the  appearance  of  a  glowing  semi- 
circular or  oval  area,  whose  centre  is  somewhat  above  the  sun  and  whose  colors 
pass  from  a  silver  white  through  a  glowing  yellow  to  a  delicate  pink  or  purple- 
rose  color,  reaching  about  twenty-five  degrees  from  the  sun. 

The  brilliancy  of  the  purple  or  rosy  light  is  greatest  when  the  sun  is  about 
four  degrees  below  the  horizon ;  its  strength  then  decreases  as  the  sun 
descends  further,  until  when  the  sun  is  six  or  seven  degrees  below  the  horizon, 
the  glow  fades  away.  In  the  very  clearest  weather  the  first  glow  is  succeeded 
by  a  second  and  fainter  glow. 

During  the  development  of  the  first  glow  a  series  of  horizon  colors  extends 
north  and  south  of  the  point  of  sunset,  increasing  for  a  time  in  strength  of 
coloring  but  at  the  same  time  decreasing  in  brightness.  These  colors  are  at 
first  yellow,  grading  rapidly  upwards  through  a  greenish  tint  to  the  blue  sky 
above,  and  fading  away  much  more  slowly  along  the  horizon  some  sixty 


44  ELEMENTARY    METEOROLOGY. 

or  eighty  degrees  distant  from  the  sun.  As  the  sun  descends,  the  yellow  belt 
close  to  the  horizon  turns  to  orange  and  then  to  red  ;  the  whole  band  narrowing 
at  the  same  time,  and  fading  when  the  depression  of  the  sun  amounts  to  six 
or  seven  degrees.  A  second  but  fainter  series  of  horizon  colors  may  accompany 
the  second  purple  light.  The  pale  western  twilight  that  remains  after  the 
disappearance  of  the  glows  and  the  horizon  colors,  is  lost  when  the  sun  is 
about  sixteen  degrees  beneath  the  horizon ;  but  the  beginning  of  dawn  occurs 
when  the  sun  is  one  or  more  degrees  further  below  our  line  of  sight. 

Accompanying  the  western  colors  of  sunset  there  is  a  series  of  well- 
marked  colors  on  the  eastern  sky.  Just  as  the  sun  reaches  the  western 
horizon,  the  eastern  horizon  is  marked  with  a  pink  band  of  color  grading 
upwards  into  blue.  As  the  sun  sinks  in  the  west,  the  pink  band  rises  in  the 
east,  in  the  form  of  a  long,  flat  arch  resting  on  the  horizon  at  points  ninety 
degrees  from  the  place  of  sunset.  Below  the  pink  band,  which  is  called  the 
twilight  arch  from  its  form  and  time  of  occurrence,  there  appears  a  belt  of 
dull  blue ;  in  clear  weather  and  level  countries  the  contrast  between  the  arch 
and  the  blue  color  beneath  it  is  very  distinct  for  some  minutes  after  sunset: 
but  with  the  rise  of  the  arch  above  the  eastern  horizon,  the  sharpness  of  its 
separation  from  the  blue  belt  fades  away,  and  on  reaching  a  height  of  from 
eight  to  twelve  degrees  it  is  hardly  perceptible. 

All  of  these  sunset  colors  are  seen  at  their  best  only  in  the  clearest 
weather.  Indeed  the  degree  of  their  development  may  be  taken  as  a  weather 
prognostic,  indicating  the  changes  of  a  day  or  two  to  come  with  considerable 
accuracy.  Turning  to  the  opposite  condition  of  more  and  more  hazy  or  turbid 
atmosphere,  we  notice  at  first  an  increase  in  the  strength  of  the  yellows  and 
reds  along  the  horizon,  and  at  the  same  time  a  decrease  in  the  distinctness  of 
the  rosy  glows.  As  the  air  becomes  more  and  more  turbid,  the  glows  dis- 
appear entirely,  and  the  horizon  colors  become  dull,  until  in  smoky  air  none 
of  the  colors  appear  except  on  the  sun  itself;  its  disc  becomes  orange,  and 
finally  deep  crimson  as  it  approaches  the  horizon ;  then  it  may  even  fade 
away  before  setting,  leaving  the  western  sky  a  dull  leaden  gray,  without  a  tint 
of  the  usual  sunset  colors,  and  the  eastern  sky  devoid  of  its  twilight  arch. 

Sunrise  is  characterized  by  a  very  similar  succession  of  colors,  but  in 
reverse  order,  and  generally  of  somewhat  fainter  tints  than  those  of  sunset. 

K.\IM,A  NATION    OF    COLOR    JN     GENERAL. 

58.  Nature  of  color.  Before  proceeding  to  the  special  explanation  of  the 
colors  of  the  atmosphere,  a  brief  statement  may  be  made  of  the  nature  and 
origin  of  colors  in  general.  It  must  be  remembered  in  the  first  place  that  tlw 
sensation  of  light  depends  upon  the  reception  in  the  eye  of  certain  undulating 
iavs  emitted  by  what  we  call  luminous  bodies,  and  transmitted  by  the  hypo- 


THE  COLORS    OF   THE   SKY.  46 

thetical  ether,  already  explained  in  Section  24.  Moreover,  luminous  bodies 
send  out  rays  of  a  great  variety  of  wave-lengths,  many  of  which  our  eyes 
cannot  perceive,  perhaps  because  the  media  of  the  eye  are  not  transparent  to 
them.  Only  the  rays  whose  wave-length  is  between  0.00036  and  0.00075  mm. 
can  be  seen.  It  is  possible  that  " seeing  light"  is  simply  the  sensation  of 
heat  produced  in  the  optic  nerves  by  the  absorption  of  the  rays  that  reach  the 
retina.  The  greater  the  amplitude  of  the  waves,  the  more  intense  the  light. 

Color  is  determined  by  the  proportion  of  rays  of  different  wave-length. 
When  the  intensity  of  the  various  rays  exists  in  the  proportions  occurring  in 
ordinary  sunlight,  we  get  no  sensation  of  color,  and  the  light  is  then  called 
white.  If  any  of  the  rays  are  unduly  intense,  or  if  others  are  unduly 
weakened,  the  light  is  colored.  A  red  light,  for  example,  is  one  in  which  the 
coarser  rays  are  more  intense;  a  blue  light,  one  in  which  the  finer  waves  are 
more  intense.  But  in  all  natural  colors  there  are  rays  of  a  great  variety  of 
wave-length,  and  the  color  that  we  perceive  is  determined  only  by  the  action 
of  the  more  intense  rays.  This  is  easily  shown  by  looking  at  a  colored  object 
through  a  prism.  The  green  of  foliage,  for  example,  is  thus  found  to  be 
merely  green  in  excess;  nearly  all  other  colors  of  the  spectrum  being  per- 
ceptible in  it.  So  the  blue  of  the  sky  or  the  red  of  sunset  contains  an  almost 
full  series  of  other  colors,  but  the  rays  which  determine  the  color  are  of  the 
greatest  intensity. 

If  sunlight  in  the  ordinary  proportions  gives  the  sensation  of  white 
light,  we  must  inquire  into  the  processes  by  which  its  normal  composition  is 
so  changed  as  to  give  the  various  colors  of  the  sky  at  one  time  and  another. 

59.  Selective  absorption  and  diffuse  reflection.  Many  solid  opaque  sub- 
stances absorb  rays  of  one  wave-length  better  than  those  of  another.  The 
rays  incident  on  such  bodies  are  thus  divided  into  two  classes;  the  one 
absorbed,  and  the  other  irregularly  turned  back  or  diffusely  reflected.1  It  : 
has  already  been  explained  in  the  chapter  on  the  temperature  of  the  atmos--. 
phere  that  the  absorbed  rays  raise  the  temperature  of  the  absorber  and 
increase  the  intensity  of  the  radiation  emitted  by  it;  but  at  ordinary  tempera- 
tures the  rays  thus  emitted  are  of  great  wave-length,  quite  imperceptible  by 
the  eye.  On  the  other  hand,  the  diffusely  reflected  rays  depart  unchanged  in 
wave-length;  but  the  proportion  of  various  wave-lengths  in  the  reflected  light 
is  greatly  altered  from  the  normal  proportions  in  the  incident  light.  The 
illuminated  object  then  appears  to  have  a  color  corresponding  to  that  of  the 
rays  that  are  in  excess.  Thus  the  green  of  foliage  is  produced;  not  that  all 
colors  but  green  are  absorbed  and  green  alone  is  reflected,  but  that  green  is 

1  It  is  probable  that  diffraction  also  takes  place  to  a  large  extent  on  the  rough  surf  ace.,  of; 
ordinary  substances;  but  the  whole  process  is  commonly  included  under  the  term,  "diffuse^ 
reflection." 


46  ELEMENTARY   METEOROLOGY. 

less  absorbed  and  more  turned  aside  than  the  other  colors.  This  process  is 
more  important  than  any  other  in  determining  the  color  of  objects  on  the 
earth's  surface;  but  it  is  not  known  to  have  application  in  causing  the  colors 
of  the  atmosphere. 

60.  Selective    absorption   and   transmission.      Transparent    substances, 
whether  solid,  liquid  or  gaseous,  are  sometimes  colorless,  sometimes  colored. 
Colorless  objects,  such  as  glass  or  water,  permit  the  nearly  free  passage  of  the 
optical  rays,  although  they  may  absorb  rays  of  other  wave-lengths.     Colored 
transparent  substances  exercise  a  selective  absorption  on  certain  of  the  optical 
rays,  allowing  the  others  to  pass,  and  thus  determine  their  color;  thus  the 
yellow  of  amber,  the  tints  of  various  inks  and  the  green  of  chlorine  gas  are 
produced.     It  is  possible  that  this  process  has  a  share  in  determining  some  of 
the  atmospheric  colors ;  but  it  does  not  control  them,  as  will  appear  further  on. 

61.  Selective  scattering.      When  a  transparent  substance,  either  solid, 
liquid  or  gaseous,  contains  suspended  in  it  a  great  number  of  excessively  fine 
particles  whose  dimensions  are  smaller  than  the  wave-lengths  of  light,  the 
rays  that  would  otherwise  be  allowed  to  pass  unobstructed  are  scattered  or 
diffracted  in  all  directions  on  every  particle;  but  the  finer  waved  rays  are 
more  effectively  turned  from  their  path  than  the  coarser  waved  rays.     The 
light  that  passes  through  such  a  turbid  medium  in  the  direction  of  the  original 
rays  therefore  becomes  more  or  less  yellow  or  red;  while  that  which  departs 
laterally  has  finer  waved  rays  in  excess,  and  appears  blue.     This  may  be  illus- 
trated by  a  simple  experiment  with  a  flask  of  soapy  water;  on  looking  through 
it  towards  a  source  of  white  light,  the  liquid  appears  somewhat  yellowish  or 
orange;  looking  across  the  direction  of  the  illuminating  rays,  the  same  liquid 
appears  to  be  bluish.     A  column  of  smoke  may  also  appear  of  different  colors, 
according  to  its  illumination.     If  looked  at  against  the  sky,  it  seems  to  be 
brownish  yellow;    if  viewed  against  a  dark  background  of  heavily  shaded 
trees,  it  appears  blue.     This  process  of  selective  scattering  is  of  much  import- 
ance in  explaining  the  colors  of  the  sky. 

62.  Diffraction  and  interference.     The  particles  in  a  turbid  medium  may 
be  of  larger  dimensions  than  the  wave  lengths  of  light.     Then  the  r;i ys  that 
are  scattered  or  diffracted  from  the  opposite  sides  of  a  single  particle  may 
"interfere"    and  extinguish  each  other.      This   process  cannot  be  explained 
here;  but  its  effects  may  be  examined  by  looking  at  a  source  of  white  lijjht 
through  a  glass  plate  on  which  lycopodiuiu  powder  is  seat  tried.     The  lijrht 
will  appear  to  be  surrounded  by  concentric  rings  of  prismatic  colors,  with  blue 
on  the  inside  and  red  on  the  outside.     If  the  diffracting  particles  are  numer- 
ous, the  rings  will  be  bright.     If  the  particles  are  all   of  one  size,  the  rings 


THE    COLORS    OF    THE   SKY.  47 

will  be  sharply  defined  with  distinct  colors.  Large  particles  produce  rings  of 
small  diameter;  small  particles  produce  large  rings.  If  the  particles  are  of 
many  sizes,  the  colors  of  the  large  and  small  rings  overlap  and  blend  into  a 
disc  of  white  light,  brighest  close  to  the  center;  the  disc  may  be  bordered 
with  a  reddish  tinge,  when  the  marginal  color  of  the  outer  ring  is  not  wholly 
lost.  Such  a  disc  may  be  called  a  diffraction  glow.  This  process  is  important 
in  explaining  certain  sunset  colors,  as  well  as  in  accounting  for  the  coronas  or 
colored  rings  around  the  sun  or  moon  in  thin  clouds. 

6&.  Refraction.  When  a  beam  of  light  passes  from  a  rarer  to  a  denser 
medium,  it  is  bent  towards  the  vertical  to  the  surface  separating  the  two 
media;  and  the  finer  waved  rays  are  bent  more  than  those  of  greater  wave 
lengths.  It  is  thus  that  white  sunlight  is  broken  or  refracted  into  the 
colors  of  the  spectrum  when  passing  through  a  transparent  prism.  We  shall 
find  application  of  this  process  chiefly  in  accounting  for  halos  and  rainbows  in 
a  later  chapter. 

EXPLANATION  OF  THE  COLORS  OF  THE  SKY. 

64.  The  dust  of  the  atmosphere.  The  general  blue  color  of  the  sky  is 
best  accounted  for  by  the  process  of  selective  scattering,  as  explained  by  Lord 
Rayleigh.  As  this  depends  on  the  presence  of  innumerable  sub-microscopic, 
non-gaseous  particles  in  the  atmosphere,  a  paragraph  may  be  given  to  their 
probable  origin  and  constitution. 

The  atmosphere  is  known  to  contain  a  vast  number  of  minute  particles, 
solid  for  the  most  part,  and  commonly  named  dust.  The  coarser  particles  will 
settle  from  a  body  of  air  if  it  is  allowed  to  rest  quietly,  and  in  speaking  of 
dust  we  commonly  refer  only  to  such  particles  as  can  easily  be  collected  from 
the  air  when  it  is  still.  But  there  is  very  good  reason  to  think  that  the  im- 
purities of  the  atmosphere  include  myriads  of  particles  vastly  finer  than  those 
to  which  the  name,  dust,  is  ordinarily  applied,  and  in  speaking  of  atmospheric 
dust  in  this  chapter,  all  particles  from  the  coarsest  to  the  finest  will  be 
included. 

The  atmosphere  receives  its  dust  chiefly  from  the  earth.  It  is  carried  up 
from  the  lands  by  the  wind;  it  is  blown  out  of  active  volcanoes;  some  of  it 
comes  from  salt  in  the  ocean,  remaining  in  the  air  when  spray  from  the  waves 
is  evaporated.  An  undetermined  share  must  come  from  the  combustion  of 
meteors  high  above  sea-level.  Water  vapor,  condensed  into  the  minutest 
drops  of  water  or  crystals  of  ice,  may  provide  much  of  the  so-called  dust. 
The  coarser  dust  for  the  most  part  being  received  from  the  land  surfaces  at 
the  bottom  of  the  atmosphere,  it  is  natural  that  its  greatest  amount  should  be 
found  in  the  lower  layers  of  air  over  the  continents;  but  it  is  borne  so  easily 


48  ELEMENTARY    METEOROLOGY. 

by  the  wind?  that  it  is  carried  far  and  wide  over  the  earth.  Vessels  in  the 
Atlantic,  west  of  the  Sahara,  may  have  their  sails  reddened  by  falling  dust 
that  has  been  carried  out  from  the  desert  by  the  trade  winds.  As  we  ascend 
above  sea-level,  even  in  regions  reputed  to  have  a  clear  atmosphere,  as  in  Italy 
or  on  the  Azores,  the  lower  strata  seem  like  a  dusty  ocean  above  which  the 
clearer  air  of  the  higher  regions  floats.  Yet  it  is  conceived  that  even  the 
upper  air  contains  innumerable  particles  of  a  size  so  small  that  in  spite  of 
their  distribution  through  the  great  volume  of  atmosphere,  their  total  quan- 
tity is  not  great,  and  their  falling  towards  the  earth  is  prevented  by  the 
faintest  ascensional  current.  Although  below  microscopic  sight,  they  must 
not  be  confused  with  the  molecules  of  the  atmospheric  gases,  which  are  to  the 
best  of  our  knowledge  of  a  far  greater  degree  of  minuteness.  The  atmosphere 
must  therefore  be  regarded,  even  when  apparently  clearest,  as  a  slightly 
turbid  medium. 

65.  The  blue  of  the  sky:  selective  scattering.  When  a  beam  of  white 
light  passes  through  the  turbid  atmosphere,  the  rays  laterally  scattered  in 
all  directions  from  the  path  of  the  beam  will  contain  a  greater  share  of  blue 
than  of  red  light.  The  coarser  the  particles,  the  greater  the  intensity  of  the 
scattered  rays,  and  the  more  uniform  the  proportion  of  the  rays  of  different 
wave-lengths.  The  finer  the  particles,  the  fainter  the  scattered  rays,  but  the 
greater  the  excess  of  blue  in  the  rays  turned  aside  from  the  original  beam. 

Now  in  looking  up  into  the  sky,  away  from  the  sun,  the  light  that  comes 
to  our  eyes  is  that  which  has  been  scattered  from  many  solar  rays  as  they 
encounter  the  myriads  of  suspended  particles  which  for  the  moment  happen 
to  be  in  our  line  of  sight;  and  as  these  particles  are  more  effective  in  turning 
aside  the  fine-waved  rays  than  the  coarser  ones,  the  eye  receives  them  in 
excess,  and  the  sky  appears  blue.1  It  is  manifest  that  the  term,  reflection, 
should  not  be  used  in  describing  this  process. 

Many  relatively  coarse  particles  in  the  lower  air  add  reflected  white  light 
to  the  blue  color  produced  by  the  smaller  particles,  and  thus  increase  the 
illumination  and  whiteness  of  the  sky.  When  the  air  is  hazy,  'the  larger 
particles  predominate,  and  the  blue  is  almost  lost  in  a  whitish  glare;  not  from 
the  cessation  of  the  process  by  which  the  blue  is  made,  but  simply  by  the 
addition  of  a  greater  and  greater  quantity  of  white  light  from  the  more, 
abundant  and  larger  particles.  When  the  air  is  very  clear,  the  action  of  the 

1  Polarization.     After  the  scattering  of  rays  on  line  particles,  their  waves  vibrate  more 
or  less  perfectly  in  a  single  plane,  instead  of  in  all  directions  transverse  to  the  ray.     The  ray 
is  then  said  to  be  polarized.     Special  instruments  are  devised  to  detect  this  peculiarity;  and 
from  these  it  is  found  that  the  light  of  the  sky  is  polarized  to  a  greater  or  less  degree;  the 
most  complete  polarization  occurring  at  an  angular  distance  of  1H)°  from  the  sun. 
found  to  be  a  necessary  consequence  of  the  scattering  of  sunlight  on  extremely  fine 
:is  has  been  shown  by  Lord  Rayleigh,  by  whom  this  theory  of  sky  color  was  ad  van 


THE   COLORS    OF    THE    SKY.  49 

finer  particles,  especially  of  those  always  present  in  the  upper  air,  produces 
the  deep  blue  of  the  celestial  vault. 

The  greater  brilliancy  and  whiteness  of  the  sky  near  the  sun  finds  its 
explanation  in  the  statement  already  made  concerning  the  greater  intensity  of 
diffraction  from  fine  particles  nearly  in  line  with  the  original  ray.  As  we  look 
closer  and  closer  towards  the  sun,  our  line  of  sight  is  more  nearly  in  the  line 
of  the  direct  rays,  and  the  intensity  of  the  light  is  correspondingly  increased. 
The  composition  of  slightly  diffracted  rays  is  so  nearly  the  same  as  that  of 
the  direct  rays  that  they  appear  like  normal  sunlight,  or  white. 

The  sky  near  the  horizon  becomes  whiter  and  paler  than  at  greater 
altitudes.  This  is  because  a  line  of  sight  passes  through  a  much  greater 
distance  of  lower  dusty  air  when  we  look  near  the  horizon  than  when  we  look 
towards  the  zenith;  the  numerous  coarse  motes  thus  encountered  turn  white 
light  to  the  eye  and  overpower  the  blue  that  comes  with  it. 

66.  The  color  of  the  sun.     The  reader  may  now  naturally  inquire  why 
the  direct  rays  of  the  sun  appear  white.     According  to  the  explanation  just 
given,  the  rays  that  come  from  the  sun  directly  to  us  have  lost  a  greater  share 
of  blue  than  of  red  rays,  and  the  sun  should  therefore  appear  of  a  reddened 
or  at  least  of  an  orange  tint.     This  reasoning  is  correct,  and  the  best  answer 
to  it  is  the  one  suggested  by  Langley.     If  we  could  see  the  sun  from  outside 
of  our  atmosphere,  it  would  probably  be  a  blue  sun;  it  appears  white  only 
after  an  excess  of  blue  in  the  original  sunbeam  has  been  turned  away  on  its 
passage  to  us  through  the  dusty  air.     In  this  respect  the  sun  is  not  a  unique 
body;    blue   stars    are   known  in  various   parts   of    the    sky,  and  these  are 
evidently  even  more  blue  than  our  sun;  otherwise  their  light  also  would  be 
white  when  it  came  down  to  us. 

67.  Deep  blue  sky  seen  from  mountains.     The  greater  purity  and  fainter 
illumination  of  the  blue  of  the  sky  when  seen  from  lofty  mountain  tops  is 
due  to  the  absence  in  the  upper  air  of  those  larger  dust  motes  so  common  at 
lower  levels.     The  larger  motes  turn  to  us  waves  of  all  kinds,  diluting  the 
blue  of  the  sky  by  the  addition  of  white  light  to  the  observer  at  sea-level. 
When  we  rise  above  the  level  at  which  the  coarser  motes  are  common,  their 
action  weakens ;  the  sky  is  less  illuminated,  but  of  a  deeper  and  purer  blue 
color;  and  at  heights  of  fifteen  or  twenty  thousand  feet  observers  describe  it 
as  of  a  deep  indigo  color,  extremely  dark  compared  to  the  well  illuminated 
sky  to  which  we  are  accustomed. 

68.  Sunset  and  sunrise  horizon  colors.     The  colors  of  evening  and  morn- 
ing are  essentially  the  same,  but  they  are  exhibited  in  reverse  order.     Those  of 
sunset  will  be  here  explained,  and  the  tints  of  sunrise  will  be  referred  to  only 
when  special  mention  of  them  is  needed. 


50  ELEMENTARY    METEOROLOGY. 

The  sunset  colors  have  already  been  divided  into  two  series;  one  arranged 
along  the  horizon, 'the  other  disposed  in  a  circular  segment  or  glow  with  the 
sun  near  the  center.  The  former  will  be  first  considered. 

The  solar  disc  itself  is  yellow  or  red  as  it  sets,  because  then  its  direct  rays 
have  traversed  so  great  a  thickness  of  air  that  the  blues  are  greatly  diminished 
by  selective  scattering,  leaving  the  others  in  excess.  As  the  sun  nears  the 
horizon,  the  lower  western  sky,  where  the  color  was  whitish  at  noon,  becomes 
yellowish,  by  reason  of  the  scattering  away  of  the  blue  rays.  When  the  sun 
is  below  the  horizon,  the  yellows  and  reds  that  fringe  the  sky-line  north  and 
south  from  the  point  of  setting,  are  for  the  most  part  due  in  the  same  way  to 
the  effective  scattering  of  the  finer  rays  in  passing  through  the  great  thick- 
ness of  air  that  they  then  must  traverse.  The  beams  of  light  that  come 
through  the  greatest  thickness  of  air,  close  to  the  horizon,  are  the  most 
strongly  reddened.  The  beams  from  a  belt  a  few  degrees  above  the  horizon 
have  passed  through  a  less  measure  of  atmosphere  and  by  a  less  direct  course, 
and  are  therefore  orange  or  yellow,  instead  of  red.  None  of  the  horizon 
colors,  however,  are  direct  rays;  they  all  suffer  more  or  less  bending  on  their 
way,  especially  those  that  come  from  points  some  distance  north  or  south  of 
the  point  of  sunset.  The  greater  their  bending,  the  less  intense  their  red 
color.  Much  of  the  bending  may  be  done  by  the  larger  dust  motes,  which 
send  white  light  in  the  day-time;  at  sunset  these  simply  send  along  the  light 
that  falls  on  them  without  affecting  its  color.  They  thus  serve  to  extend  the 
reds  and  yellows  along  the  western  horizon,  but  not  to.  alter  the  color  of  the 
light  that  falls  on  them. 

If  the  observer  stands  on  a  lofty  mountain  peak  overlooking  a  broad 
region  of  much  lower  level,  the  horizon  reds  are  intensified.  This  is  due  to 
the  more  complete  scattering  away  of  the  blue  rays  in  the  additional  measure 
of  dusty  air  traversed  by  the  horizon  beams,  which  to  the  observer  on  the 
lowlands  have  to  pass  through  a  less  distance. 

Refraction  has  a  small  share  in  producing  sunset  colors.  When  the  rays 
of  light  enter  the  atmosphere  obliquely,  they  are  bent  or  refracted  from  their 
path  towards  the  denser  lower  air.  An  observer  always  sees  the  sun  at  a 
greater  altitude  above  the  horizon  than  it  really  is.  When  the  sun  appears  to 
be  on  the  horizon  line,  it  has  really  passed  below  the  horizon.  Refraction  is 
greatest  at  the  horizon,  when  it  increases  the  apparent  altitude  of  a  celestial 
object  by  about  half  a  degree.  The  sun's  angular  diameter  being  of  the  same 
measure,  it  follows  that  the  lower  limb  or  edge  of  the  solar  disc  will  seem  to 
rest  on  the  horizon  when  the  upper  limb  is  really  just  passing  bdmv  it. 

The  longer-waved  rays  are  refracted  less  than  the  finer-waved  ones;  a  star 
seen  in  a  telescope  near  the  horizon  exhibits  the  prismatic  colors,  with  red 
below  and  blue  above.  After  sunset  every  solar  Ix-ani  will  he  similarly  broken 
into  a  short  vertical  spectrum.  The  successive  spectra  will  overlap,  and  aid 


THE    COLORS    OF    THE    SKY.  51 

the  process  of  selective  scattering  in  producing  a  gradation  from  red  on  the 
horizon  through  yellows  to  blue  in  the  upper  sky;  but  of  the  two  processes, 
the  scattering  is  the  more  effective. 

It  does  not  appear  warrantable  to  attribute  any  considerable  share  of 
atmospheric  colors  to  selective  absorption  and  transmission.  There  is  no 
sufficient  direct  evidence  to  show  that  either  dry  air  or  water  vapor  are 
colored,  in  the  sense  that  chlorine  is  colored.  The  variation  in  the  intensity 
of  the  sunset  colors  proves  that  they  cannot  be  due  to  absorption  by  the  air 
itself.  If  it  be  assumed  that  the  red  of  sunset  is  caused  by  absorption  of  the 
blue  rays  by  water  vapor,  then  the  blue  of  the  open  day-time  sky  remains  to 
be  explained.  Moreover,  while  the  intensity  of  sunset  reds  increases  with  the 
dampness  of  the  lower  atmosphere,  it  does  not  show  a  close  dependence  on 
the  absolute  amount  of  water  vapor  present ;  the  colors  vary  with  the 
approach  of  the  vapor  to  the  condition  of  saturation  and  condensation.  It 
therefore  seems  more  reasonable  to  disregard  the  absorptive  effects  of  water 
vapor  in  the  gaseous  state,  and  consider  only  the  action  of  minute  particles  of 
condensed  water  or  ice  in  aiding  other  kinds  of  suspended  matter  to  cause 
sunset  colors  by  scattering  the  solar  rays. 

69.  The  twilight  arch.  The  colors  on  the  horizon  opposite  the  sun  are 
also  best  explained  by  selective  scattering.  At  sunset  the  pink  band  along 
the  eastern  horizon,  which  rises  and  forms  the  twilight  arch  as  the  sun 
descends,  is  the  return  to  us  of  the  excessively  red  light  by  which  the  eastern 
sky  is  then  illuminated;  the  light  being  red  when  it  reaches  us,  it  becomes 
redder  still  as  it  goes  further  on,  and  is  then  returned  as  a  faint  illumination 
from  the  particles  that  it  encounters.  It  should  be  recalled  in  this  connection 
that  the  backward  scattering  of  light  is  symmetrical  with  the  forward  scatter- 
ing; and  that  the  red  rays  are  returned  backward  in  greater  force  than  the 
blues,  which  are  for  the  most  part  thrown  off  laterally.  As  the  light  from 
the  western  horizon  is  red  as  it  passes  us,  it  is  still  more  reddened,  although 
diminished  in  intensity,  by  the  selective  scattering  that  returns  it  from  the 
eastern  sky.  At  the  same  time  many  irregular  scatterings  give  us  blue  light 
with  the  red,  and  thus  make  the  arch  of  a  rosy  color,  rather  than  an  intense 
red  as  in  the  west.  A  similar  rosy  color  is  seen  opposite  sunset  on  the  snow 
of  mountains  or  on  lofty  clouds,  as  in  the  rear  of  a  thunder-storm  retreating 
in  the  east  after  sunset;  the  snow  or  cloud  does  not  produce  the  red  color,  but 
simply  sends  back  to  us  the  color  that  falls  on  it. 

The  dark  bluish  area  beneath  the  rising  twilight  arch  is  the  shadow  of  the 
earth  on  the  sky.  The  rays  of  light  from  the  sun  cannot  reach  this  portion 
of  the  sky  without  many  turnings  and  scatterings  on  the  way,  so  that  any 
light  coming  back  to  the  eye  from  the  shaded  sky  has  lost  nearly  all  its  red, 
and  hence  appears  of  a  distinct  blue  color.  The  under  edge  of  the  arch  is 


62  ELEMENTARY    METEOROLOGY. 

distinct  at  first,  but  becomes  blurred  as  it  rises.  This  is  because  our  line  of 
sight  after  sunset  departs  more  and  more  from  the  surface  which  separates  the 
arch  and  shadow.  Just  after  sunset  we  stand  almost  in  the  surface  of  separa- 
tion and  look  closely  along  it ;  then  there  is  a  sharp  separation  between  the 
colors  above  and  below  it :  but  as  the  rotation  of  the  earth  carries  us  into  the 
shadow,  we  look  across  the  surface  and  our  line  of  sight  then  receives  rays 
from  both  the  blue  shadow  and  from  the  rosy  area ;  hence  the  colors  are 
blended  and  their  separation  fades  away. 

Subordinate  effects  of  the  same  kind  as  the  blue  shadow  of  the  earth  are 
seen  in  shadows  of  clouds  and  mountains  on  the  sky.  When  the  western 
sky  contains  massive  clouds  at  sunset,  the  eastern  twilight  arch  will  be 
distinctly  interrupted  by  delicate  bluish  rays,  whose  narrow  lower  ends  all 
converge  to  a  point  on  the  edge  of  the  arch  opposite  to  the  sun;  the  con- 
vergence being  an  effect  of  perspective  on  really  parallel  cloud  shadows.  In 
the  same  way,  if  an  observer  stand  upon  a  lofty  mountain  at  sunset,  he  will 
see  the  shadow  of  the  mountain  rising  above  the  eastern  horizon  and  interrupt- 
ing the  twilight  arch.  The  shadows  of  adjacent  peaks  are  also  sometimes 
seen,  but  less  distinctly.  The  isolated  summit  of  .  Pikes  Peak  in  the  Rocky 
Mountains,  or  of  Fujiyama  in  Japan,  casts  an  immense  solitary  conical  blue 
shadow  on  the  sky  at  sunrise  or  sunset. 

70,  Sunset  and  sunrise  glows.  The  white  or  yellow  oval  glow,  changing 
to  rose  or  purple  as  it  fades  away  after  sunset,  is  accounted  for  by  the  inter- 
ference of  solar  rays  scattered  or  diffracted  from  particles  of  various  sizes  in 
the  lower  and  upper  air. 

The  bright  white  glow  around  the  sun  at  noon  is  essentially  a  diffraction 
glow  on  particles  of  many  sizes.  Near  sunset  the  solar  rays  pass  through  so 
great  a  thickness  of  air  and  encounter  so  many  more  particles  than  at  noon 
that  the  disc  becomes  brighter  and  much  larger;  but  the  particles  encountered 
are  still  of  so  many  different  sizes  that  no  distinct  color  is  produced. 

Shortly  after  sunset,  when  the  observer  and  the  air  for  several  thousand 
feet  above  him  are  in  the  shadow  of  the  earth,  the  glow  comes  only  from 
particles  in  the  upper  air;  and  as  these  are  small  and  of  more  nearly  uniform 
size  than  those  near  the  earth,  the  glow  increases  still  more  in  purity  of  color 
as  the  lower  air  darkens,  and  the  delicate  rosy  marginal  color  makes  its 
appearance  at  an  angular  distance  of  20  or  25  degrees  from  the  sun.  The 
color  is  not  brilliant,  but  in  the  waning  twilight  it  is  clearly  seen;  in  lim- 
weather  it  constitutes  one  of  the  chief  glories  of  the  western  sunset  sky.  It 
descends  and  fades  away  when  the  sun  is  about  six  degrees  below  the  horizon; 
to  be  followed  in  the  clearest  weather  by  a  much  fainter  rosy  or  purple  after- 
glow, visible  for  a  short  time.  This  is  best  explained  as  a  second  ring  of  the 
same  origin  as  the  first,  but  at  a  greater  angular  distance  from  the  sun.  Its 


THE    COLORS    OF   THE   SKY.  53 

suggested  explanation  by  reflection  is  not  satisfactory,  because  there  are  no 
particles  in  the  lofty  layers  of  the  atmosphere  then  illuminated  that  are  large 
enough  to  cause  reflection;  they  can  only  diffract  the  light  that  falls  on  them. 
A  moderate  haze  in  the  lower  air  weakens  or  obscures  the  rosy  glows.  They 
are  therefore  in  general  better  seen  in  winter  than  in  summer. 

71.  The  red  sunsets  of  1883-84.  In  1883,  '84,  '85,  the  glows  obtained  an 
extraordinary  development,  which  is  believed  to  have  resulted  from  the 
presence  then  in  the  upper  air  of  minute  particles  of  dust  or  of  condensed 
vapor,  blown  out  of  the  volcano,  Krakatoa,  in  the  Strait  of  Sunda,  between 
Java  and  Sumatra,  late  in  August,  1883.  At  the  same  time  the  sun  was  sur- 
rounded even  at  noon  on  clear  days  by  a  dusky  reddish  ring  of  about  20° 
radius,  known  as  Bishop's  ring,  after  its  first  observer;  this  being  the  day- 
time appearance  of  the  diffractive  sunset  glow,  then  so  brilliant  as  to  be 
visible  in  the  fully  illuminated  sky,  but  usually  of  faint  intensity,  so  that  it 
can  be  seen  only  after  sunset. 

These  brilliant  sunsets  attracted  great  attention  at. the  time,  and  the  records 
of  their  appearance  in  different  parts  of  the  world  have  been  carefully  studied. 
The  eruption  occurred  with  excessive  violence  on  August  26  and  27,  1883, 
destroying  half  of  the  island  of  Krakatoa,  leaving  water  more  than  a  thousand 
feet  deep  where  the  volcano  had  stood  before,  and  shaking  the  air  so  vigorously 
as  to  produce  an  atmospheric  wave  that  broke  windows  a  hundred  miles  away 
and  travelled  around  the  •  earth,  converging  at  the  antipodal  point  and  then 
returning  to  its  source ;  the  automatic  barometric  records  kept  in  different 
]  >arts  of  the  world  indicate  that  the  wave  went  out  from  Krakatoa  and  back 
from  the  antipodal  point  at  least  three  times.  The  sounds  of  the  explosion 
were  heard  over  the  Malay  archipelago,  half  of  Australia  and  half  of  the 
Indian  ocean,  even  three  thousand  miles  away.  The  sea-waves  driven  away 
from  the  bursting  volcano  caused  great  destruction  on  the  coasts  of  the  neigh- 
boring islands,  drowning  over  30,000  persons,  and  then  swept  across  the 
oceans,  registering  their  arrival  on  tide-gauges  in  various  harbors  in  all  parts 
of  the  world.  Pumice  and  dust  blown  from  the  volcano  blackened  the  sky 
and  fell  for  hundreds  of  miles  around,  obstructing  the  sea.  The  finer  dust 
and  the  icy  particles  condensed  from  the  ejected  vapor,  whose  sudden  expan- 
sion is  believed  to  have  caused  the  explosion  of  the  volcano,  reached  great 
altitudes  in  the  atmosphere,  and  there  spread  around  the  world.  As  the  dust 
spread  over  the  sky,  the  sunset  colors  became  extraordinarily  brilliant,  the 
usually  faint  second  glow  restoring  vivid  colors  to  the  fading  sky  and  exciting 
remark  from  all  observers.  The  dates  of  the  first  occurrence  of  these  striking 
phenomena  in  different  parts  of  the  world  have  been  carefully  charted,  and  it 
is  thus  seen  that  they  spread  rapidly  westward  from  Krakatoa  around  the 
equator,  completing  the  circuit  of  the  earth  in  fifteen  days,  and  then  gradually 


54  ELEMENTARY    METEOROLOGY. 

spreading  poleward.  Before  the  end  of  the  year  they  were  visible  in  all  parts 
of  the  world.  Their  duration  extended  through  the  greater  part  of  1884  and 
into  1885,  and  the  reddish  ring  around  the  sun  was  seen  even  in  1886.  An 
excessive  fineness  of  the  suspended  particles  is  thus  indicated. 

Brilliant  sunsets  were  recorded  in  1783  and  in  1831,  following  great 
volcanic  eruptions  in  those  years  ;  many  other  less  remarkable  examples  of 
the  same  relation  of  sunset  colors  to  volcanic  eruptions  have  been  noted.  In 
recording  such  phenomena,  it  is  important  to  note  the  dates  of  first  visibility 
of  the  vivid  colors,  the  time  of  the  appearance  and  change  of  the  successive 
colors,  and  their  altitude  above  the  horizon. 

While  an  excess  of  very  fine  particles  in  the  upper  air  increases  the 
intensity  of  the  sunset  colors,  an  excess  of  coarser  dust  in  the  lower  air 
reduces  their  brightness,  and  may  even  conceal  them  entirely.  The  delicate 
rosy  glow  is  the  first  to  be  extinguished  in  this  way,  and  as  the  turbidity 
increases  even  the  stronger  horizon  tints  fail  to  appear.  The  direct  rays  from 
the  solar  disc  are  the  last  to  disappear ;  only  the  most  intense  red  rays  then 
reach  the  observer,  leaving  the  sky  a  dull  dead  gray  all  around :  sometimes 
the  sun  itself  disappears  in  the  hazy  or  smoky  sky  before  it  sets,  even  though 
no  clouds  are  present. 

A  peculiar  instance  of  the  dependence  of  sky  colors  upon  the  dustiness  of 
air  was  observed  by  the  author  several  years  ago  in  Cambridge,  Mass.  In  the 
afternoon  a  brief  squall  of  dry  wind  blew  a  great  quantity  of  dust  into  the 
air.  The  sunset  was  devoid  of  colors  except  in  the  sun  itself,  which  dis- 
appeared on  approaching  the  horizon  as  a  circle  of  deep  crimson  color.  The 
cloudless  sky  was  dull,  and  even  as  darkness  came  on,  few  stars  appeared. 
The  half  moon,  well  up  in  the  sky,  shone  out  with  a  very  unusual  red  color  ; 
at  the  same  time  some  of  the  brighter  stars  appeared  faintly,  and  with  a 
reddish  tinge.  In  the  course  of  a  few  hours  the  quantity  of  dust  was  so 
greatly  diminished,  either  by  settling  down  or  drifting  away,  that  the  stars 
appeared  in  the  usual  number  and  the  moon  lost  its  red  color,  passing  gradu- 
ally through  orange  to  its  normal  tint. 

72.  Mirage  and  looming.  Further  account  may  be  given  here  of  certain 
peculiar  effects  of  reflection  on  atmospheric  layers  of  different  density,  of 
which  brief  mention  was  made  in  Section  46.  Rays  of  light  may  be  totally 
reflected  at  the  surface  of  contact  of  two  layers  of  unlike  density,  if  the  angle 
of  incidence  is  very  large.  Hence  when  a  layer  of  very  warm  air  lies  close  to 
the  surface  of  a  plain,  the  eye  of  an  observer  who  stands  above  this  layer 
receives  from  the  further  parts  of  it  only  the  reflected  light  of  the  sky  or  of 
elevated  objects  in  the  distance,  and  no  rays  from  the  ground  beneath. 
Objects  thus  reflected  are  inverted,  as  if  from  a  horizontal  mirror  whose,  plane 
is  below  the  observer;  such  an  appearance  bein«j  called  a  mirage.  It  is  com- 


THE    COLORS    OF    Till:    SKY. 


55 


mon  in  calm  weather  and  in  the  hot  hours  of  the  day  on  level  desert  surfaces, 
and  also  over  water  surfaces  when  a  light  wind  from  the  land  carries  out  air 
of  a  temperature  unlike  that  of  the  water.  A  slight  change  in  the  height  of 
the  observer  may  cause  a  considerable  change  in  the  mirage ;  and  if  no  mirage 
is  seen  a  few  feet  above  the  ground  or  water,  it  may  often  be  discovered  at  a 
less  height.  Some  slight  mirage  is  nearly  always  visible  when  the  eye  is 
within  an  inch  or  two  of  an  extended  surface  of  quiet  water. 

Over  the  sea  in  the  neighborhood  of  the  coast,  and  particularly  in  the 
Arctic  regions,  it  often  happens  that  the  surface  of  reflection  is  above  the 
observer.  The  reflection  is  then  called  looming,  and  is  characterized  by  an 
inverted  image  above  the  object.  The  image  is  often  elongated  vertically, 
producing  an  appearance  of  spires  and  pinnacles  of  fantastic  form.  Objects 
that  are  below  the  observer's  horizon  may  be  thus  brought  to  view. 


ELEMENTARY   METEOROLOGY. 


CHAPTER  V. 


THE    MEASUREMENT    AND    DISTRIBUTION    OF    ATMOSPHERIC 

TEMPERATURES. 
THERMOMETRY. 

73.  Thermometers.  The  explanations  in  the  third  chapter  of  the  processes 
by  which  the  air  is  warmed  and  cooled  prepare  us  to  take  up  the  study  of 
thermometry,  or  the  determination  of  the  temperature  of  the  air,  with  good 
unde  rs  tanding. 

A  thermometer,  or  heat-measure,  is  an  instrument  by  which  the  temperature 
of  a  body  may  be  compared  with  certain  adopted  standards  of  temperature. 

The  standards  are  the  freezing  and  boiling 
points  of  pure  water,  boiling  to  take  place 
under  one  atmosphere  of  pressure.  For  con- 
venience of  measurement,  the  intermediate 
temperatures  between  these  wide  extremes  are 
graded,  or  divided  into  degrees  ;  but,  unfortun- 
ately, the  different  countries  of  the  world  have 
not  adopted  degrees  of  the  same  value.  The 
scale  of  the  Fahrenheit  thermometer,  commonly 
employed  in  this  country  and  in  Great  Britain 
and  her  colonies,  begins  to  count  its  degrees  at 
a  temperature  not  naturally  defined  ;  and  places 
freezing  at  32°  and  boiling  at  212°.  One  hun- 
dred and  eighty  degrees,  therefore,  correspond 
on  this  scale  to  the  difference  between  the 
temperatures  of  freezing  and  boiling  water. 
The  centigrade  thermometer  places  its  zero 
point  at  freezing  and  100°  at  boiling  ;  100 
centigrade  degrees,  therefore,  equal  180  Fahren- 
heit degrees.  When  temperatures  below  the  zero  point  are  observed,  they 
should  be  recorded  with  a  minus  sign,  thus  :  —  0°,  and  read  "  six  degrees  below 
zero." 

The  essentials  of  a  good  thermometer  may  be  summarized  as  follows  :  The 
liquid  in  its  tube  must  not  freeze  at  any  temperature  that  it  is  likely  to 
experience.  ^The  volume  of  the  bulb  must  be  large  in  comparison  with  Unit 
of  the  tube,  in  order  to  render  any  change  of  volume  by  expansion  in  the  bulb 
easily  apparent  in  the  greater  length  of  the  column  in  the  tube.  The  tube 
must  be  of  constant  diameter,  in  order  that  equal  increments  of  temperature 


FIG.  7. 


MEASUREMENT  OF  ATMOSPHERIC  TEMPERATURES.         57 

shall  produce  equal  increments  of  length  in  the  column,  jthe  scale  should  be 
etched  or  cut  on  the  tube  itself.  s  The  temperatures  indicated  by  the  graduation 
of  the  scale  should  have  been  carefully  determined  by  comparison  with  accurate 
standards,  and  the  intermediate  divisions  of  the  scale  must  be  accurately 
marked  at  equal  intervals. 

It  is  a  waste  of  labor  to  undertake  observations  of  temperature  with  an 
inaccurate  thermometer.  A  great  deal  of  labor  has  unfortunately  been  wasted 
in  this  way.  Many  of  the  records  of  temperature  that  have  been  kept  for 
years  with  great  patience  are  worse  than  useless,  because,  being  inaccurate, 
they  are  misleading.  A  good  thermometer  costs  but  two  or  three  dollars  ;  if 
carefully  used,  it  may  last  many  years.  But  even  a  good  thermometer  will 
not  give  good  records  unless  it  is  properly  exposed.  Persons  who  desire  to 
undertake  regular  meteorological  observations  should  therefore  apply  to  their 
local  State  Weather  Service,  or  to  the  Weather  Bureau  in  Washington,  for  full 
instructions  concerning  instruments,  shelters,  records,  etc. 

It  is  desired  that  a  thermometer,  when  read,  shall  indicate  the  temperature 
of  the  open  air  at  a  small  height  over  the  ground,  and  not  so  close  to  buildings 
as  to  be  affected  by  them.  For  this  reason,  the  thermometer  should  be  placed 
in  a  shelter,  with  protection  from  sunshine  and  rain  or  snow,  but  well  open  to 
the  wind.  It  should  be  removed,  if  possible,  from  buildings  and  trees.  When 
this  is  not  possible,  a  small  shelter  placed  on  the  shady  side  of  a  building,  not 
too  high  above  the  ground,  may  serve ;  but  it  is  not  worth  while  to  attempt  to 
make  a  record  of  temperature  unless  the  exposure  is  such  as  will  warrant 
confidence  in  the  records.  In  cities,  a  shelter  on  the  roof  of  a  building  is 
probably  better  than  at  a  window  in  a  narrow  street.  It  is  to  be  expected 
that  the  temperature  of  cities  should  be  somewhat  higher  than  that  of  the 
surrounding  open  country. 

74.  The  sling  thermometer.  In  case  no  satisfactory  shelter  can  be 
provided,  correct  records  can  be  obtained  by  tying  the  •thermometer  to  a  string 
two  or  three  feet  long  and  whirling  it  around  in  the  air  until  its  reading  does 
not  change.  The  instrument  thus  arranged  is  called  the  sling  thermometer. 
It  is  of  especial  use  in  exploring  expeditions,  or  in  local  studies  of  the  varia- 
tions of  temperature  in  a  small  district,  where  no  proper  shelter  can  be  counted 
on,  and  it  should  be  frequently  employed  when  establishing  a  permanent 
shelter  for  meteorological  instruments,  in  order  to  determine  the  difference 
l)ct\veen  its  temperature  and  that  of  the  surrounding  air,  particularly  in  quiet 
sunny  weather. 

Records  of  temperature  should  be  carefully  entered  in  a  book  kept  for  that 
purpose.  At  the  beginning  of  the  record  a  careful  statement  should  be  made 
describing  the  thermometer  and  its  location ;  any  subsequent  change  of 
exposure  should  be  clearly  entered  at  its  proper  date. 


58  ELEMENTARY    METEOROLOGY. 

75.  Thermographs.  The  temperature  of  the  air  is  continually  changing 
at  a  more  or  less  rapid  rate.  A  perfect  record  would  present  a  complete  indi- 
cation of  all  the  changes.  This  cannot  be  gained  by  ordinary  thermometers, 
but  it  is  practically  gained  by  self-recording  instruments,  known  as  thermo- 
(//•(i/Jis,  Two  styles  of  thermographs  are  illustrated  in  the  accompanying 
figures. 

The  Draper  self-recording  thermometer,  or  thermograph,  an  American 
instrument  (Fig.  8),  possesses  a  metallic  thermometer,  one  end  of  which  is  fixed 
while  the  other  end  is  attached  to  a  train  of  levers,  to 
magnify  the  small  movements  due  to  expansion  or  con- 
traction by  change  of  temperature.  The  end  of  the  last 
lever  carries  a  pen  which  contains  a  non-freezing  glycerine 
ink,  and  rests  on  a  circular  record  sheet  that  rotates  once 
a  week.  As  the  temperature  rises,  the  pen  is  carried  out- 
wards from  the  center  of  the  sheet;  as  the  temperature 
falls,  the  pen  is  carried  inwards.  The  sheet  is  divided 
into  days  and  hours  by  curved  radial  lines,  and  into  de- 
grees by  concentric  circular  lines  ;  so  that  the  temperature 
at  any  time  can  be  easily  read  off.  Although  instruments 
of  this  kind  are  not  so  accurate  as  good  mercurial  ther- 
mometers, they  make  up  for  their  slight  inaccuracy  by  the  continuity  of  their 
record ;  and  if  checked  by  frequent  readings  of  a  mercurial  thermometer  and 
driven  by  an  accurate  clock,  they  are  of  great  value.  The  Draper  self-recording 
thermometer  is  made  by  the  Draper  Manufacturing  Co.,  152  Front  Street, 
New  York.  The  cost  of  the  instrument  is  $15.00. 

The  Richard  Freres  thermograph  (Fig.  9),  made  in  Paris  (Glaenzer  &  Co., 
80  Chambers  Street,  New  York,  are  agents  for  this  country),  is  of  a  somewhat 
different  pattern,  costing,  without  duty,  about  $25.00.  The  thermometer  is 
here  a  flat  bent  tube  of  brass,  containing  a  non-freezing  liquid.  One  end  of 
the  tube  is  fixed  to  the  frame  of  the  instrument ;  the  other  end  moves  freely 
with  change  of  temperature,  and  works  a  train  of  levers,  which,  as  before, 
magnify  the  movement  of  the  tube.  If  the  temperature  rises,  the  greater 
expansion  of  the  liquid  than  of  the  tube  bends  the  tube  towards  a  straight 
line ;  if  the  temperature  falls,  the  elasticity  of  the  tube  bends  it  into  a  sharper 
curve.  The  pen  at  the  end  of  the  last  lever  bears  lightly  on  a  sheet  of  paper 
that  is  wrapped  around  a  drum  or  barrel ;  the  drum  is  turned  around  once  a 
week  (or  once  a  day,  if  so  ordered)  by  clock-work  within  it.  The  pen  rises 
and  falls  with  the  temperature  and  thus  writes  its  record  on  the  rotating  drum. 
Sample  curves  from  sheets  of  this  pattern  are  given  on  reduced  scale  in  the 
adjoining  figures,  10,  11,  12.  Curve  a,  Fig.  10,  illustrates  a  period  of  clear 
warming  weather  from  April  27  to  30,  1889,  with  large  and  regular  diurnal 
range,  from  a  record  kept  by  the  Jackson  Company  at  Nashua,  N.  H.,  for  the 


MKASfllKMENT    OF    ATMOSPHERIC    TEMPERATURES. 


59 


England  Meteorological  Society  ;  curve  b  presents  the  effects  of  a  spell  of 
cloudy  weather  at  Nashua  accompanying  the  passage  of  the  great  Hatteras 
hurricane  of  September  13  to  16,  1889  ;  curve  c  shows  the  change  from  a  time 


FIG.  D. 

of  moderate  winter  weather  to  a  cold  spell  at  Nashua,  February  22  to  25, 
1889,  the  change  occurring  at  midnight  of  the  23—24  ;  curve  d  exhibits  a 
steady  fall  of  temperature  from  the  night  of  one  day  over  the  next  neon  to  the 
following  night,  in  the  coming  of  a  winter  cold  spell  at  Nashua,  January  19  to 
21,  1889  ;  curve  e  is  the  reverse  of  the  preceding  case,  being  the  effect  of  an 


6     A'oon   6        12       6     Noon    6        12       6     Noon     6       12       6     Noon    « 


10. 


approaching  mild  spell  at  Nashua,  December  16  and  17,  1888,  in  which  there 
was  a  continuous  rise  of  temperature  through  a  night  from  noon  to  noon ; 
curve  /  illustrates  especially  well  the  value  of  thermograph  records,  in  giving 


60 


ELEM ENTARY   METEOROL<  H ;  Y. 


90 


the  occurrence  of  a  nocturnal  temperature  maximum  caused  by  warm  southerl y 
winds  followed  by  cold  winds  from  the  west,  at  Cambridge,  Mass.,  November 
30  and  December  1,  1890  ;  curve  g  shows  the  sudden  rise  of  temperature 
on  the  coming  of  a  chinook  wind  (Section  248)  at  Fort  Assiniboine,  Mont., 

January  19,  1892.  Curve  a,  Fig.  11,  from 
the  office  of  the  City  Engineer  at  Provi- 
dence, R.  I.,  records  the  passage  of  a  vio- 
lent thunder-squall  just  after  noon,  July 
21, 1885  ;  while  the  afternoon  maximum  of 
the  following  day  possesses  small  oscilla- 
tions, ascribed  to  local  convectional  air 
currents.  Curve  b  illustrates  the  effects 
of  cool  diurnal  sea-breezes  at  Cambridge, 
Mass.,  May  13  and  14,  1887,  by  which  the 
apex  of  the  curve  is  truncated  about  noon-day.  Fig.  12  contains  records  from 
the  Harvard  College  Observatory  for  Denver  and  Pikes  Peak,  Colo.,  for  August 
19,  1887  (a,  £),  and  for  March  3,  1888  (c,  d)  ;  the  mountain  temperature  beinq- 
the  lower  in  the  summer  example  ;  but  the  higher  for  a  few  hours  in  this  par- 
ticular winter  example.  The  peculiar  features 
of  these  records  would  be  lost  in  observations 
taken  at  fixed  hours  two  or  three  times  a  day. 

76.  Maximum  and  minimum  thermometers. 
When  thermographs  cannot  be  employed,  other 
devices  are  introduced  in  order  to  secure  special 
records  iii  the  simplest  way.  The  most  import- 
ant of  these  are  seen  in  the  maximum  and  mini- 
mum thermometers,  shown  in  Fig.  13.  These 
instruments  register  the  highest  and  lowest 
temperatures  of  the  day.  The  maximum  ther- 
mometer, the  lower  one  in  the  figure,  has  a 
narrow  constriction  in  the  tube  just  outside  of 
the  bulb.  As  the  temperature  rises,  the  mercury 
is  driven  out  of  the  bulb ;  but  as  the  tempera- 
ture falls,  the  mercury  does  not  return  ;  the 
upper  end  of  the  column  in  the  tube  therefore  registers  the  highest  reading 
since  the  instrument  was  set.  Setting  is  done  by  whirling  the  instrument 
rapidly  on  a  peg  at  its  head,  when  the  mercury  is  driven  back  past  tli« 
constriction  into  the  bulb.  The  minimum  thermometer  contains  a  transparent 
non-freezing  liquid,  instead  of  mercury.  A  small,  double-headed  pin  or  index 
li«-s  inside  of  the  liquid  in  the  tube  (between  60°  and  70°  in  the  figure).  By 
raising  the  bulb,  the  index  slides  along  to  the  end  of  the  liquid  column  ;  the 


MEASUREMENT    OF    ATMOSPHERIC    TEMPERATURES.  61 

instrument  is  then  left,  slanting  gently  towards  the  bulb  j  as  the  temperature 
rises,  the  expansion  of  the  liquid  is  too  slow  to  push  the  index  along  with  it ; 
but  when  the  temperature  falls  below  that  at  which  the  instrument  was  set, 
the  surface  cohesion  of  the  liquid  carries  the  index  down  the  tube.  The 
position  in  which  the  upper  end  of  the  index  lies  therefore  registers  the 
lowest  or  minimum  reading  since  the  previous  setting.  A  pair  of  good  maxi- 
mum and  minimum  thermometers  costs  about  $6.00  ;  they  are  very  easily 


FIG.  13. 

cared  for ;  a  single  observation,  as  late  in  the  evening  as  possible,  sufficing  to 
determine  the  diurnal  range  of  temperature,  which  is  one  of  the  most  important 
weather  elements.  The  time  at  which  the  highest  and  lowest  temperatures 
occurred  are,  however,  not  indicated.  The  readings  of  the  maximum  and 
minimum  thermometers  should  always  be  entered  in  the  record  book  before 
the  instruments  are  set  for  a  new  observation. 

77.  Black-bulb  thermometer.     It  is  sometimes  desired  to  obtain  an  indi- 
cation of  the  intensity  of  sunshine,  independent  of  the  temperature  of  the 
air.     This  is  roughly  effected  by  exposing  a  maximum  thermometer  having 
the  bulb  coated  with  dull  lamp-black,  the  thermometer  being  enclosed  in  a 
glass  tube  from  which  the  air  has  been  exhausted.     The  lamp-black  on  the 
bulb  absorbs  a  large  share  of  the  sunshine,  and  the  absence  of  air  around  the 
bulb  prevents  cooling  by  conduction.     A  temperature  much  above  that  of  the 
surrounding  air  is  thus  reached.     It  is  customary  to  record  simply  the  excess 
of  the  maximum  thus  gained  over  that  of  the  ordinary  maximum  reading. 
This  excess,  however,  varies  so  greatly  with  the  conditions  surrounding  the 
instrument  that  it  is  not  admissible  to  regard  observations  with  black-bulb 
thermometers  as  having  any  precise  or  comparable  value.     The  instrument 
cannot  be  recommended  for  ordinary  observations. 

78.  Record  of  temperature:  mean  temperatures.     Besides  the  record  of 
the  temperature  of  the  air  at  certain  times,  it  is  desired  to  determine  also  the 
mean  temperature  of  the  air  for  the  day,  the  months  and  the  year.     This 
could  be  done  by  making  hourly  records  and  calculating  their  mean  for  each 
day  ;  the  average  of  the  successive  diurnal  means  would  give  the  mean  of  the 
month  ;  and  the  average  of  the  monthly  means,  the  mean  of  the  year.     The 


62  ELEMENTARY  METEOROLOGY. 

average  of  twenty  or  thirty  successive  yearly  means  suffices  to  establish  the 
mean  temperature  of  a  place. 

It  is  manifest,  however,  that  few  persons  can  maintain  a  long  series  of 
hourly  observations.  These  are  sometimes  taken  at  the  larger  observatories  ; 
but  they  are  now  mostly  superseded  by  thermographs,  checked  by  maximum 
and  minimum  thermometers.  The  question  then  arises,  at  what  several  hours 
of  the  day  shall  ordinary  observations  be  taken  in  order  that  their  average 
shall  give  a  close  measure  of  the  true  diurnal  mean  ?  This  is  determined  by 
the  hourly  observations  that  have  been  made  at  certain  stations.  First,  the 
mean  temperatures  of  the  successive  hours,  1  o'clock,  2  o'clock,  3  o'clock,  etc., 
are  separately  determined  for  every  month.  Selection  is  then  made  of  certain 
pairs  or  groups  of  hours  whose  mean  corresponds  closely  to,  or  differs  by  a 
small  number  of  degrees  from  the  true  diurnal  mean.  While  the  mean  of 
two,  three  or  four  observations  in  a  day  at  the  hours  thus  chosen  cannot  be 
trusted  to  determine  the  true  mean  of  any  single  day,  yet  if  the  observations 
are  continued  for  a  month,  they  will  serve  to  determine  accurately  enough  the 
monthly  mean,  and  then  from  successive  monthly  means  the  annual  mean  may 
be  derived.  Hours  thus  recommended  for  observation  are  as  follows  :  —  For 
two  daily  records,  hours  of  the  same  name  in  the  morning  and  afternoon,  as 
7  a.  m.  and  7  p.  m.;  8  a.  m.  and  8  p.  in.,  etc.;  for  three  daily  records,  6  a.  m., 
2  and  9  p.  m. ;  or  7  a.  m.,  2  and  9  p.  in.  (add  double  the  reading  at  9  p.  in.  to 
the  readings  at  the  other  hours  and  divide  the  sum  by  four).  The  mean  of 
the  maximum  and  minimum  records  is  often  used,  but  it  is  from  a  half  to  one 
degree  too  high. 

None  of  these  combinations  have  a  greater  error  than  a  degree  or  a  degree 
and  a  half  in  denning  the  monthly  mean ;  and  the  error  of  the  annual  mean 
is  still  less.  Tables  published  by  the  Weather  Bureau  and  by  the  Smithsonian 
Institution  at  Washington  give  the  corrections  by  which  the  means  thus 
determined  can  be  best  reduced  to  the  true  means  ;  and  when  thus  corrected, 
the  monthly  and  annual  results  may  be  trusted  to  a  small  fraction  of  a  degree. 

79.  Climatic  data.  Besides  the  means  already  mentioned,  it  is  customary 
to  determine  certain  other  data  in  defining  the  climate  of  a  place.  The  most 
important  are: — Monthly  and  annual  means;  mean  diurnal  range  for  each 
month;  means  of  successive  five-day  periods  or  pentads  through  the  year 
for  single  years,  and  for  the  same  periods  in  groups  of  five  years,  or 
lustra;  means  of  five-year  periods,  or  lustra,  beginning  with  years  whose 
dates  end  with  1  or  6 ;  absolute  extremes  of  temperature  for  every  month  ; 
mean  of  the  monthly  extremes  for  successive  years ;  average  difference 
between  successive  daily  means. 

When  observations  of  good  thermometers,  well  exposed  and  regularly  read, 
extend  over  a  lustrum  period,  they  should  be  thus  reduced,  and  the  results 


MEASI  I;I:.MI:N  r  OF  ATMOSPHERIC  TEMPERATURES.  63 

published  in  the  reports  of  the  local  State  Weather  Services  (Sect.  323)  as 
contributions  to  local  climatology. 

In  a  region  whose  mean  annual  temperature  is  as  variable  as  in  the  central 
and  eastern  United  States,  it  will  be  found  that  the  values  of  successive  lustra 
do  not  agree  precisely.  A  much  longer  period  than  five  years  is  needed  for 
the  determination  of  the  true  mean  annual  temperature.  It  is  therefore 
inuulvisable  to  compare  the  mean  temperatures  of  adjacent  stations  if  the 
periods  of  observation  do  not  agree.  For  example,  the  mean  temperature  of 
New  Bedford,  Mass.,  for  the  lustrum  beginning  with  1836  was  47°.0 ;  that  of 
Providence,  .R.  I.,  for  the  lustrum  beginning  1846  was  48°. 8  :  from  which  it 
would  appear  that  the  mean  of  the  latter  was  1°.8  higher  than  that  of  the 
former.  But  in  the  lustrum  beginning  in  1861,  New  Bedford  had  a  mean  of 
49°. 9  ;  and  in  the  lustrum  beginning  in  1836,  Providence  had  a  mean  of  46°. 7 : 
and  from  this  it  would  appear  that  the  former  was  3°.2  above  the  latter.  For 
the  period  1836-1876,  the  two  means  differ  less  than  half  a  degree. 

Whenever  possible,  climatic  comparisons  of  temperature  or  other  data 
should  be  made  for  identical  periods.  If  the  observations  for  the  region  con- 
cerned do  not  cover  the  same  period,  it  is  desirable  that  they  should  be  reduced 
to  a  definite  period,  such  as  an  interval  from  1870  to  1890.  This  can  be  done 
approximately,  as  follows :  —  A  certain  station,  S,  may  have  records  of  tem- 
perature from  1855  to  1875 :  determine  the  mean  for  this  period  at  adjacent 
stations  of  long  continued  observation:  determine  the  average  difference 
between  these  means  and  the  means  for  the  period  1870-1890 :  apply  this 
average  difference  as  a  correction  to  the  mean  of  station  S  ;  and  the  result  will 
be  the  probable  mean  for  its  locality  for  the  period  1870-1890. 

80.  Isothermal  charts.  Observations  of  temperature  have  been  maintained 
at  many  stations  in  all  parts  of  the  world,  some  of  them  for  all  the  years  of 
this  century,  but  generally  for  shorter  periods  ;  the  distribution  of  tempera- 
ture over  the  earth  is  studied  by  means  of  such  records.  In  order  to  make 
observations  taken  at  different  altitudes  on  land  comparable  with  one  another, 
it  is  customary  to  reduce  all  temperatures  to  sea-level  by  adding  one  degree  to 
the  annual  or  monthly  mean  for  every  three  hundred  feet  of  altitude  ;  but 
in  preparing  daily  weather  maps,  the  actual  thermometer  readings  are 
charted. 

Observations  made  at  sea  are  gathered  by  the  Hydrographic  Offices  of 
various  countries  ;  they  are  classified  first  by  position,  all  of  those  taken 
within  a  certain  "square"  of  latitude  and  longitude  being  placed  together; 
and  again  by  months.  The  records  are  then  reduced  to  the  true  mean  of  the 
month  as  carefully  as  possible.  As  the  variation  of  temperature  at  sea  is 
generally  much  smaller  and  more  regular  than  on  land,  the  results  thus 
obtained  are  fairly  accurate,  particularly  in  those  parts  of  the  ocean  wher? 


64  ELEMENTARY  METEOROLOGY. 

many  vessels  pass.  The  North  Atlantic  is  especially  well  known  in  this 
respect.  The  "square"  bounded  by  20°  and  25°  longitude  west  of  Greenwich 
and  by  0°  and  5°  north  latitude  has  10,329  observations  for  March  on  the 
meteorological  charts  published  by  our  Hydrographic  Office  at  Washington. 

When  observations  are  gathered  and  reduced  for  many  stations  in  various 
parts  of  the  world,  they  may  be  charted  on  maps  for  the  better  illustration  of 
the  distribution  of  temperature  ;  either  for  the  mean  annual  temperature  or 
for  the  mean  of  the  seasons  or  of  the  months.  Lines  may  then  be  drawn 
through  places  estimated  to  have  mean  temperatures  of  50°,  60°,  70°,  and  so  on ; 
such  lines  are  called  isotherms,  or  lines  of  equal  temperature.  An  isothermal 
chart  thus  constructed  shows  at  a  glance  the  areas  that  are  on  the  average 
warmer  or  colder  than  any  given  temperature. 

The  best  general  isothermal  charts  of  the  world  are  those  prepared  by 
Dr.  Julius  Hann  of  Vienna,  and  by  Professor  Alexander  Buchan  of  Edin- 
burgh. The  former  are  published  in  Berghaus'  Physical  Atlas  J  (1887) ;  they 
present  the  isotherms  on  the  centigrade  scale.  The  latter  include  a  beautiful 
series  of  monthly  isothermal  charts  on  the  Fahrenheit  scale,  published  in 
1889  by  the  British  government  to  illustrate  an  essay  on  the  Atmospheric 
Circulation  in  the  Report  on  the  Challenger  Expedition ;  but  their  high  cost 
places  them  beyond  general  use.  The  isothermal  charts  on  a  small  scale  here 
presented  are  reduced  from  certain  ones  of  this  series  ;2  the  following  sections 
call  attention  to  the  chief  facts  to  be  learned'  from  them. 

DISTRIBUTION  OF  TEMPERATURE  OVER  THE  EARTH. 

81.  Contrast  between  the  equator  and  poles.  The  most  general  facts 
presented  by  the  isothermal  chart  for  the  year  (Chart  I)  are  the  familiar  high 
temperatures  around  the  equator  and  the  low  temperatures  about  the  poles. 
The  sufficient  reason  for  this  has  already  been  found  in  the  greater  annual 
value  of  insolation  at  the  equator,  decreasing  to  smaller  values  at  the  poles. 
The  line  of  highest  mean  annual  temperature,  which  may  be  called  the  mean 
annual  heat  equator,  is  not  of  uniform  temperature  all  around  its  circuit.  Its 
temperatures  are  five  or  more  degrees  higher  on  the  lands  than  on  the  oceans. 
At  the  first  glance  one  might  explain  this  as  the  result  of  the  lower  specific 
heat  of  the  land  and  of  its  non-volatile  character :  but  as  the  inequality  appears 
in  the  mean  annual  temperature,  this  explanation  will  not  hold.  It  is  true 
that  if  the  mean  temperature  of  the  daytime  or  of  the  summer  only  wen- 
charted,  the  air  over  the  lands  would  then  be  found  on  the  average  of  higher 
temperature  than  that  over  the  ocean  for  the  above  reasons  ;  but  as  the  mean 
for  the  year  includes  the  conditions  for  ni^hi  as  well  as  for  day,  and  for  winter 

1  The  meteorological  section  of  this  atlas,  containing  1'J  charts,  may  be  bought  separately. 

2  These  charts  are  on  Gall's  projection,   in  which  the  distortion  of  high  latitudes  is  less 
than  in  Mercator's  projection,  commonly  employe.!. 


DISTRIBUTION    OF    ATMOSPHERIC    TEMPERATURES.  65 

as  w«'ll  as  for  summer,  the  rapid  cooling  of  the  land  and  of  the  air  close  to  it 
at  the  colder  times  must  counterbalance  the  rapid  heating  in  the  warmer  times  ; 
and  hence  for  the  mean  of  the  year  there  should  not  be,  for  the  suggested 
reasons,  any  higher  temperature  on  the  heat  equator  over  the  lands  than  over 
the  oceans. 

The  true  cause  of  the  varying  temperature  along  the  heat  equator  is  to  be 
found  in  the  interchange  of  polar  and  equatorial  waters  by  the  ocean  currents, 
whereby  the  equatorial  ocean  is  somewhat  cooled  and  the  polar  oceans  are 
much  warmed ;  while  on  the  lands  there  is  no  such  interchanging  process. 
The  torrid  lands  are  therefore  hotter  than  the  ocean  of  the  same  latitude ; 
and  the  lands  of  high  latitudes  are  colder  than  seas  alongside  of  them.  The 
lands  take  a  temperature  proper  to  their  latitude,  while  the  oceans  attempt  to 
equalize  the  temperatures  between  equator  and  poles. 

A  marked  consequence  of  this  is  seen  in  the  more  rapid  decrease  of 
temperature  from  the  mean  annual  heat  equator  towards  the  poles  on  land 
than  on  water  ;  in  other  words,  the  poleward  temperature  gradient  is  stronger 
on  the  continents  than  on  the  oceans.  Beginning,  for  example,  in  southern 
India  and  tracing  a  line  almost  northward  to  the  Arctic  coast  of  northeastern 
Siberia,  the  temperature  falls  from  85°  to  0°,  a  decrease  of  almost  a  degree 
and  a  half  of  temperature  in  a  degree  of  latitude.  Following  a  northward  line 
of  the  same  length  in  the  Atlantic  ocean,  the  decrease  is  from  83°  to  25°,  or  at 
a  rate  of  a  degree  of  temperature  to  a  degree  of  latitude. 

82.  Irregularity  of  annual  isotherms.  The  explanation  that  has  already 
been  given  of  the  distribution  of  insolation  over  the  earth  might  lead  us  to 
expect  that  the  mean  annual  isotherms  should  coincide  with  the  lines  of 
latitude.  A  glance  at  the  map  shows  that  in  many  parts  of  the  world  the 
isotherms  are  unsymmetrical  in  the  two  hemispheres  and  that  they  depart 
greatly  from  an  east  and  west  course.  "We  will  first  consider  the  character  of 
the  departures  and  then  look  for  their  explanation. 

The  unsymmetrical  arrangement  of  the  isotherms  on  either  side  of  the 
geographical  equator  is  first  seen  in  the  location  of  the  heat  equator  in  the 
northern  hemisphere  for  the  greatest  part  of  its  circuit,  as  may  be  shown 
by  drawing  a  line  bisecting  the  space  between  the  pairs  of  corresponding 
isotherms  of  the  torrid  zone  in  either  hemisphere.  This  line  falls  into 
southern  latitudes  only  in  the  western  part  of  the  Pacific  and  in  Australasia ; 
its  location  here  being  due  to  a  southern  movement  of  the  warm  equatorial 
waters  in  that  region,  and  to  the  higher  mean  temperatures  of  the  land  areas 
of  Australasia  than  of  the  water  areas  on  the  opposite  side  of  the  equator. 
Elsewhere,  the  heat  equator  lies  in  northern  latitudes.  The  absence  of  any 
southern  continent  to  balance  the  effect  of  Asia  explains  its  course  across  the 
northern  Indian  ocean  ;  and,  as  will  be  seen  in  a  later  section,  the  inflow  of 


66  ELEMENTARY    METEOROLOGY. 

cool  waters  from  the  far  southern  latitudes  displaces  the  line  to  the  north  of 
the  equator  in  the  eastern  Atlantic  and  Pacific. 

In  the  next  place,  the  departures  are  small  in  the  southern  hemisphere, 
where  the  isotherms  are  remarkably  regular,  especially  on  the  broad  oceanic- 
areas.  The  lines  are  much  more  irregular  in  the  northern  hemisphere  ;  but  in 
both  hemispheres  there  are  certain  systematic  deflections  of  isotherms  from 
the  parallels  of  latitude  in  passing  from  an  ocean  over  a  continent  to  the  next 
ocean.  In  crossing  eastward  over  South  America,  for  example,  the  lines  turn 
equatorward  on  the  Pacific  near  the  western  coast ;  they  loop  strongly  pole- 
ward in  crossing  the  land,  and  finally  run  slowly  towards  the  equator  again  in 
traversing  the  Atlantic.  A  similar  irregularity,  but  not  so  pronounced,  is  seen 
in  the  passage  of  the  lines  over  Africa  and  Australia.  Coming  now  to  our 
hemisphere,  the  isotherms  on  the  Pacific  turn  somewhat  towards  the  equator 
in  the  middle  and  lower  latitudes  as  they  approach  North  America  ;  then 
entering  the  continent,  they  loop  poleward  ;  and.  on  reaching  the  Atlantic  they 
turn  slowly  toward  the  equator  on  their  way  across  to  Africa.  A  similar 
irregularity  may  be  perceived,  but  less  distinctly,  on  the  broad  lands  of  the 
old  world  in  equivalent  latitudes. 

In  the  higher  latitudes  of  the  northern  hemisphere  the  isotherms  show  just 
the  opposite  deflections.  They  turn  towards  the  pole  in  the  northeast  Pacific, 
towards  the  equator  in  northeast  America,  strongly  towards  the  pole  in  the 
northeast  Atlantic,  and  so  on.  There  is  no  land  far  enough  south  in  the 
other  hemisphere  to  exhibit  deflections  of  this  kind,  except  a  small  part  of 
South  America. 

As  a  result  of  all  this,  the  isotherms  are  crowded  together  on  the  eastern 
coasts  of  the  northern  continents,  and  spread  far  apart  on  the  eastern  side  of 
the  northern  oceans.  This  is  particularly  apparent  on  the  two  sides  of  the 
Atlantic,  where  it  is  of  especial  interest,  because  the  lands  on  either  side  of 
this  ocean  are  at  present  the  seat  of  the  highest  civilization  in  the  world.  In 
western  Europe,  one  may  travel  a  thousand  miles  northward  without  finding 
so  great  a  change  of  mean  annual  temperature  as  would  be  found  in  a  voyage 
of  half  that  distance  along  our  eastern  coast. 

The  reason  for  the  systematic  deflections  of  the  isotherms  is  found  almost 
entirely  in  the  even  more  systematic  course  of  the  great  ocean  currents.  The 
interchange  of  ocean  waters  between  the  equator  and  poles  already  mentioned 
is  performed  in  part  by  a  surface  flow  towards  the  poles  and  a  slow  creeping 
of  the  deep  waters  back  again  towards  the  equator  ;  but  there  is  besides  this 
an  eddy -like  circulation  of  the  surface  waters  in  the  several  oceans  which  is  of 
much  more  importance  in  meteorology;  because  the  temperature  of  the  air, 
which  we  are  now  discussing,  depends  so  largely  on  that  of  the  surface  on 
which  it  rests.  The  eddy -like  currents  of  the  ocean  may  now  be  simply 
generalized. 


DISTRIBUTION    OF    ATMOSPHERIC    TEMPERATURES.  67 

83.  General  scheme  of  ocean  currents.  The  North  Pacific  is  the  simplest 
of  all  the  oceans  from  having  practically  no  connection  with  the  adjacent  polar 
waters.  Its  great  eddy  turns  from  left  to  right,  consisting  of  an  equatorial 
portion  moving  from  east  to  west ;  of  a  western  portion,  known  as  the  Japanese 
current,  which  passes  the  Japanese  Islands  northeastward  ;  a  northern  portion 
traversing  the  ocean  towards  Alaska ;  and  an  eastern  portion  flowing  along 
our  western  coast  to  the  southeast,  completing  the  circuit.  There  is  a  notice- 
able subordinate  eddy  turning  around  from  right  to  left  in  the  Bay  of  Alaska, 
and  a  small,  cold  southward  current  from  Kamchatka  towards  Japan.  No 
significant  supply  of  cold  water  comes  from  the  Arctic  ocean  through  Bering 
strait  to  the  Pacific. 

The  South  Pacific  ocean  has  a  similar  general  eddy  of  rather  greater  size  ; 
but  its  circulation  is  from  right  to  left ;  its  western  portion  flows  among  the 
Polynesian  islands  and  near  New  Zealand  and  Australia  ;  it  gives  off  a  small 
branch  north  of  Australia  to  the  Indian  ocean  ;  its  southern  portion  is  con- 
fluent with  the  great  Antarctic  eddy  which  runs  from  west  to  east  around  the 
south  pole.  The  member  of  the  Pacific  eddy  that  flows  equatorward  along 
the  western  coast  of  South  America  is  known  as  the  Peruvian  or  Humboldt 
current ;  it  furnishes  a  greater  share  of  cool  water  to  the  equator  than  is 
brought  by  any  other  current.  The  two  equatorial  portions  of  the  North  and 
South  Pacific  eddies  are  not  perfectly  confluent,  but  are  separated  by  a  some- 
what irregular  counter-current,  running  from  west  to  east  a  little  north  of  the 
equator,  and  carrying  a  body  of  warm  water  to  the  coast  of  Central  America. 

The  eddy  of  the  South  Indian  ocean  is  similar  to  that  of  the  South 
Pacific  in  being  confluent  with  the  great  Antarctic  eddy  on  its  polar  side. 
Contrary  to  the  representation  generally  given  on  maps  of  ocean  currents,  it 
does  not  give  out  a  branch  to  the  South  Atlantic  around  the  southern  end  of 
Africa.  The  currents  of  the  Northern  Indian  ocean  are  anomalous  in  chang- 
ing their  course  with  the  seasons  :  in  the  northern  summer  they  possess  a 
normal  left-to-right  eddy,  whose  equatorial  portion  is  then  confluent  with  the 
corresponding  portion  of  the  South  Indian  eddy  ;  in  the  southern  summer 
the  eddy  turns  the  other  way,  so  that  its  equatorial  portion  then  corresponds 
to  an  equatorial  counter-current. 

The  South  Atlantic  eddy  is  also  confluent  with  the  Antarctic  eddy  on  its 
polar  side,  but  it  is  strongly  unlike  the  eddies  of  the  other  southern  oceans  in 
giving  out  a  great  branch  that  flows  obliquely  across  the  equator  into  the 
northern  hemisphere.  This  is  the  result  of  the  unsymmetrical  form  of  Africa 
and  South  America,  the  former  extending  to  the  west,  north  of  the  equator  ; 
the  latter  extending  to  the  east,  south  of  the  equator.  The  southern  hemi- 
sphere loses  and  the  northern  hemisphere  gains  a  large  volume  of  warmed 
water,  by  this  peculiar  arrangement  of  continents  and  ocean.  Mention  should 
be  made  of  a  cold  current  that  wedges  along  the  eastern  side  of  Patagonia 


68  ELEMENTARY    METEOROLOGY. 

towards  the  equator,  the  only  distinct  cold  current  on  the  eastern  side  of  a 
southern  continent. 

The  North  Atlantic  possesses  the  most  peculiar  system  of  currents  of  all 
the  oceans.  Its  normal  eddy  receives  on  the  southern  side  the  great  branch 
given  off  by  the  South  Atlantic  eddy,  and  in  turn  it  gives  off  from  its  northern 
side  a  great  volume  of  water  which  passes  northward  beyond  Norway,  circles 
around  the  great  gulf  commonly  called  the  Arctic  ocean,  and  returns  greatly 
chilled  to  supply  the  cold  Labrador  current  which  creeps  down  our  eastern 
coast  as  far  as  Cape  Hatteras.  A  small  counter  current  of  variable  extent 
flows  eastward  between  the  North  and  South  Atlantic  eddies  into  the  Gulf  of 
Guinea  ;  its  greatest  extension  coming  in  the  late  northern  summer.  This  is 
closely  analogous  to  the  counter  current  of  the  equatorial  Pacific. 

In  the  ordinary  nomenclature  of  the  currents  of  the  North  Atlantic,  undue 
emphasis  is  given  to  that  portion  which  is  supposed  to  come  from  the  Gulf  of 
Mexico.  It  is  true  that  a  considerable  volume  of  warm  water  issues  from  the 
Gulf  between  Florida  and  Cuba  ;  the  name  "Gulf  Stream "  should  be  limited 
to  this  concentrated  current.  But  it  cannot  be  believed.  4that  all  the  warm 
water  which  flows  northward  off  our  eastern  coast,  and  then  drifts  eastward 
and  northeastward  towards  Europe,  has  issued  from  this  moderate  source.  A 
considerable  portion  of  it  must  have  passed  outside  of  the  West  India  islands, 
without  making  the  side  circuit  of  the  Caribbean  sea  and  the  Gulf  of  Mexico. 

It  should  be  noticed  particularly  that  while  the  deformity  of  the  North 
Pacific  eddy  on  the  north  consists  only  in  the  subordinate  eddies  by  Alaska 
and  north  of  Japan,  the  North  Atlantic  eddy  gives  off  a  great  branch  on  the 
north  which  makes  the  circuit  of  the  Polar  sea. 

84.  Deflection  of  isotherms  by  ocean  currents.  Bearing  in  mind  that  the 
equatorial  waters  are  warm  and  the  polar  waters  are  cold,  it  follows  that 
currents  from  the  equator  will  tend  to  warm  the  air  and  deflect  the  isotherms 
towards  the  poles,  while  the  currents  returning  from  higher  latitudes  will 
carry  the  isotherms  equatorwards.  The  deflections  of  the  isotherms  already 
described  can  all  be  accounted  for  by  this  principle. 

The  spreading  apart  of  the  isotherms  on  the  western  side  of  North 
America,  for  example,  is  due  to  the  opposite  courses  of  the  return  current  that 
passes  along  California  and  Mexico  in  lower  latitudes,  and  the  subordinate 
eddy  that  circles  from  right  to  left  around  the  Alaskan  Bay  in  higher  lati- 
tudes. Similarly  opposite  courses  of  ocean  currents  on  a  much  larger  scaL- 
are  found  in  the  eastern  North  Atlantic,  and  hence  the  divergence  of  the, 
isotherms  on  the  western  coasts  of  Europe  and  northern  Africa  is  still  mm-e 
marked;  the  equatorward  deflections  in  !<>\v  latitudes  being  controlled  by  the 
eastern  portion  of  the  normal  North  Atlantic  eddy  past  Spain  and  the  western 
Sahara,  while  the  excessive  northward  deflections  in  the  higher  latitudes  are 


m  ?m^ 


DISTRIBUTION    OF    ATMOSPHERIC    TEMPERATURES.  69 

due  to  the  great  Arctic  branch  of  the  North  Atlantic  eddy,  so  peculiar  to  this 
ocean  and  commonly  considered  an  extension  of  the  Gulf  Stream. 

The  crowding  of  the  isotherms  on  the  eastern  side  of  Asia  depends  in  the 
lower  latitudes  on  the  northward  turn  of  the  Japanese  current,  and  in  the 
higher  latitudes  on  the  southward  turn  of  the  subordinate  current  between 
Kamchatka  and  Japan.  The  crowding  of  the  lines  east  of  North  America  is 
more  pronounced,  because  the  currents  which  control  the  isotherms  there  are 
of  so  remarkable  a  strength.  Off  the  coasts  of  Florida  and  Carolina  the 
isotherms  are  held  to  the  north  by  the  Gulf  Stream  proper ;  along  the  coast 
of  New  England  and  the  Provinces  they  are  carried  strongly  to  the  south  by 
the  powerful  Labrador  current. 

In  the  southern  hemisphere  none  of  the  continents  offer  serious  interrup- 
tion to  the  eastward  course  of  the  great  Antarctic  eddy  ;  hence  no  strong 
deflections  of  the  isotherms  are  found  in  high  southern  latitudes.  Nearer  the 
equator  the  deflections  are  similar  to  those  of  low  latitudes  in  the  northern 
hemisphere.  The  peculiar  deflections  of  the  isotherms  in  the  northern  hemi- 
sphere and  their  comparative  regularity  in  the  southern  are  thus  well 
accounted  for. 

An  interesting  corollary  may  be  drawn  from  these  explanations.  If  our 
earth  possessed  a  surface  of  level  land  only,  the  difference  of  temperature 
between  the  equator  and  poles  would  be  much  greater  than  it  is  at  present,  even 
greater  than  on  the  existing  lands  where  the  poleward  decrease  of  temperature 
is  comparatively  rapid;  while  on  a  world  of  continuous  water  surface,  if  there 
existed  a  gradual  interchange  between  equatorial  and  polar  waters,  such  as 
might  be  reasonably  expected,  the  contrast  of  equatorial  and  polar  temperatures 
would  be  smaller.  In  either  case  the  distribution  of  temperature  all  over  the 
world  would  be  as  regular  as  we  now  find  it  in  the  southern  hemisphere. 

85.  Isotherms  for  January  and  July.  We  may  next  examine  Charts  II 
and  III,  representing  the  mean  temperatures  of  the  earth  in  January  and 
in  July.  The  general  variation  from  winter  to  summer  in  either  hemisphere  is 
simply  enough  explained  by  referring  to  the  variations  in  the  values  of  insola- 
tion with  the  seasons,  as  given  in  Section  27.  It  should  be  recalled  in  this 
connection  that  in  January  the  earth  is  nearest  to  the  sun ;  hence  if  distance 
from  the  sun  alone  controlled  our  temperatures,  we  should  expect  to  find  the 
southern  summer,  which  occurs  in  perihelion,  of  higher  temperature  as  a 
whole  than  the  northern  summer,  which  occurs  in  aphelion.  Comparing  the 
summers  of  the  two  hemispheres,  we  find  that  the  reverse  is  true.  The  sum- 
mer of  the  southern  hemisphere  in  January  is  marked  by  moderately  high 
temperatures  ;  the  summer  of  the  northern  hemisphere  in  July  possesses 
excessive  heats  over  large  areas.  The  location  of  the  latter  areas  being  all  on 
the  land,  we  readily  discover  the  reason  for  this  perhaps  unexpected  result. 


70 


ELEMUNTAKY    METEOROLOGY. 


Although  the  sunshine  is  truly  stronger  during  the  southern  summer  on 
account  of  our  nearness  then  to  the  sun,  so  great  a  share  of  the  southern 
hemisphere  possesses  a  water  surface  that  the  temperature,  even  under 
stronger  sunshine,  cannot  rise  to  a  high  degree  :  in  the  northern  summer,  in 
spite  of  the  slightly  weaker  sunshine  consequent  upon  our  greater  distance 
from  the  sun,  the  temperatures  produced  are  high,  because  it  is  so  easy  to 
raise  the  temperature  of  the  land  surface,  and  upon  this  the  temperature  of 
the  air  depends. 

It  may  be  also  noted  that  if  we  take  the  mean  temperature  of  the  earth  as 
a  whole,  it  is  not  constant  throughout  the  year.  It  might  at  first  be  expected 
that  a  slightly  higher  mean  temperature  should  prevail  in  perihelion  than  in 
aphelion ;  and  this  would  be  true  if  the  surface  of  the  earth  .were  of  but  one 
kind,  or  if  the  land  and  water  were  symmetrically  distributed  on  either  side 
of  the  equator ;  but  under  existing  conditions  it  is  found  that  the  average 


FIG.  14  (January). 


FIG.  15  (July). 


temperature  of  the  earth  as  a  whole  is  higher  in  July,  63°,  when  the  northern 
hemisphere  is  excessively  hot  and  the  southern  hemisphere  but  moderately 
cold,  than  in  January,  55°,  when  the  southern  hemisphere  is  moderately  warm 
and  the  northern  hemisphere  is  excessively  cold. 

Just  as  the  summer  of  the  northern  or  land  hemisphere  is  warmer  and  tin1 
winter  is  colder  than  the  same  seasons  of  the  southern  or  water  hemisphere, 
so  the  various  continental  areas  are  warmer  in  summer  and  colder  in  winter 
than  the  adjacent  oceans  of  similar  latitude.  This  is  strikingly  apparent  in 
the  case  of  Asia,  where  the  excessive  heat  of  Arabia,  Persia  and  northern 
India  in  summer  and  the  excessive  cold  of  Siberia  in  winter  offer  the  extreme 
<  xamples  of  terrestrial  temperature  variation.  Similar  variations  but  of  a 
more  moderate  range  are  found  in  Australia.  Kven  the  interior  of  (Jreat, 
P.ritain  is  warmer  than  the  surrounding  sea  in  summer  and  colder  in  winter; 
and  peninsular  Spain  and  Portugal  exhibit  winter  and  summer  isotherms 
roughly  following  the  coast  line,  as  in  l-'i^s.  1  1  ami  l."i. 


DISTRIBUTION    OF   ATMOSPHERIC    TEMPERATURES.  71 

86.  Poleward  temperature  gradients  in  winter.     Looking  again  at  the 
general  rate  of  decrease  of  temperature  from  equator  to  pole,  it  will  be  seen 
that  this  is  stronger  in  the  winter  hemisphere  than  in  the  summer  hemisphere ; 
particularly  if  the  comparison  is  made  between  the  northern  winter  and  the 
southern  summer.     The  change  in  the  value  of  this  temperature  gradient  is 
not  well  marked  in  the  southern  hemisphere,  because  there  the  change  of 
temperature  with  the  seasons  is  comparatively  small ;  but  in  the  northern 
hemisphere  it  is  very  distinct.     In  the  summer  time  the  great  area  of  lands  in 
higli  northern  latitudes  determines  the  occurrence  of  comparatively  high  tem- 
peratures in  the  far  north ;   hence  the  poleward  decrease  of  temperature  at 
this  time  is  comparatively  gradual ;  but  in  the  winter  time  the  great  area  of 
northern  lands  allows  the  temperature  to  fall  excessively  low,  and  the  decrease 
of  temperature  from  the  equator  towards  the  north  pole  is  extremely  rapid. 
This  will  be  found  to  be  of  importance  in  explaining  the  more  violent  winds 
of  our  winter  season. 

87.  Migration  of  isotherms.     The  migration  of  the  sun  north  and  south 
of  the  equator,  by  reason  of  the  obliquity  of  the  earth's  axis  to  the  plane  of 
its  orbit,  causes  a  migration  of  the  heat  equator  also ;  but  while  the  sun  shifts 
its  position  from  23^-°  north  to  23^-°  .south  of  the  equator,  the  heat  equator 
generally  migrates  by  a  much  less  amount.     Moreover,  while  the  sun  stands 
farthest  north  or  south  of  the  equator  in  June  or  December,  the  greatest 
migration  of  the  heat  equator  northward  or  southward  is  found  in  July  or 
August  and  January  or  February  ;  just  as  the  hottest  part  of  the  day  is  an 
hour  or  two  after  noon.     The  shifting  of  the  heat  equator  is  particularly 
small  on  the  oceans.     On  the  Pacific  it  moves  over  15°  or  20°  of  latitude  ;  on 
the  Atlantic  still  less,  and  only  on  the  western  part  of  this  ocean  does  it  cross 
to  the  southern  hemisphere  when  the  sun  is  south  ;  being  held  elsewhere  north 
of  the  equator  by  the  cool  African  current :  a  notable  consequence  of  this  will 
be  found  in  Section  225.     On  the  continents  it  shifts  over  a  greater  distance. 
In  Africa  it  moves  from  about  23°  N".  to  20°  S. ;  this  migration  being  almost 
symmetrical  north  and  south,  because  the  land  there  extends  about  equally  on 
either  side  of  the  equator.     In  America  the  migration  is  from  35°  N.  to  15°  S. 
A  more  peculiar  case  is  found  in  the  Indian  ocean  and  on  the  land  to  the  north 
of  it.     When  the  sun  comes  north  of  the  equator,  the  heat  equator  runs 
beyond  it  to  the  deserts  of  Persia  in  latitude  33°  X.     In  the  opposite  season, 
when  the  sun  goes  south,  the  heat  equator  hangs  behind  it  and  reaches  only 
10°  S.  latitude,  because  there  are  no  southern  lands  in  these  longitudes  to  tempt 
it  further ;  in  Australia,  however,  it  reaches  latitude  20°  S.     Important  con- 
sequences of  this  unsymmetrical  migration  will  be  found  in  the  chapters  on 
the  winds  and  rainfall. 


72 


ELEMENTARY    METEOROLOGY. 


An  interesting  study  may  be  made  of  the  migration  of  any  intermediate 
isotherm  of  the  temperate  zones.  On  the  oceans  of  the  southern  hemisphere, 
the  isotherm  of  50°  shifts  annually  from  latitude  35°  or  40°  to  45°  or  50°.  In 
the  middle  of  the  North  Pacific  the  shift  of  the  50°  isotherm  is  from  latitude 
43°  to  53°.  On  the  axis  of  North  America  the  same  line  migrates  from 
Arkansas,  latitude  33°,  almost  to  the  Arctic-  shore  of  British  America,  in 
latitude  67°  ;  and  over  Asia  it  travels  from  latitude  28°  in  southeastern  China 
to  latitude  70°  near  the  Lena  delta  in  Siberia. 

88.  Northern  winter  isotherms.  The  irregularities  in  the  course  of  the 
isotherms  already  examined  on  the  chart  for  the  year  are  exceeded  by  those 
found  on  the  chart  for  our  northern  winter.  The  lands  tend  to  take  tempera- 


120  160 


40  .        80 


ISANOMALOUS 
TEMPEKATCKE  LINES 

i    FOR  JANIJARY , 


FIG.  16. 

tures  proportionate  to  their  latitudes  and  proper  to  their  season,  while  the 
waters  try  to  maintain  temperatures  of  the  preceding  summer,  and  the  influx 
of  equatorial  waters  greatly  aids  the  northern  seas  in  this  attempt.  The 
isotherms  in  our  winter  seasons  are  therefore  extraordinarily  deflected, 
particularly  on  the  two  sides  of  the  North  Atlantic ;  the  isotherms  are 
carried  far  to  the  south  along  our  eastern  const,  and  far  to  the  north  along 
the  western  coast  of  Europe.  In  Lapland,  the  lines  even  lean  over  on  their 
backs. 

89.  Thermal  anomalies.  The  difference  between  the  mean  temperature  of 
any  place  and  the  mean  temperature  of  its  latitude  is.  called  its  thermal 
anomaly.  The  anomalies  for  January  and  July  are  illustrated  in  Figs.  16  and 


DISTRIBUTION   OF    ATMOSPHERIC    TEMPERATURES. 


73 


17.1     Areas  not  differing  more  than  two  degrees  from  the  mean  of  their 
latitude  are  shaded  with  vertical  lines. 

For  January  the  northeastern  Atlantic  and  northwestern  Europe  are 
regions  of  excessive  warmth  for  their  latitude,  being  about  35°  F.  in  excess  of 
their  normal.  Northeastern  Siberia  is  the  region  of  the  most  excessive  cold 
iu  the  world,  having  a  January  temperature  of  30°  below  its  normal.  The 
waters  of  the  Alaskan  Bay  and  the  broken  lands  north  of  Hudson's  Bay  are 
the  districts  of  too  high  and  too  low  temperatures  in  the  new  world,  their 
departures  being  about  20°  above  and  25°  below  their  respective  normals. 
<  hily  small  anomalies  are  found  in  the  torrid  and  south  temperate  zones,  as 
might  have  been  foreseen  from  the  regular  course  of  their  isotherms. 


ISANOMALOUS 
TEMPERATURE  LINES 

FOR  JULY 


FIG.  17. 

The  July  anomalies  are  less  pronounced  than  those  of  January.  The 
northern  continents  are  warmer  and  the  oceans  are  colder  than  the  normals  of 
their  latitudes,  but  the  departures  are  not  so  strong  as  before  ;  excessive  heat, 
10°  or  more  above  the  normal,  is  found  from  eastern  Siberia  to  the  Sahara  and 
in  our  western  interior  deserts  of  Nevada  and  Arizona. 

The  annual  anomalies  exhibit  positive  and  negative  departures  in  the 
northern  hemisphere  similar  to  those  of  January,  but  less  marked.  A  notable 

1  The  charts  from  which  these  figures  are  reduced  were  constructed  on  the  basis  of 
Buchan's  isothermal  charts  by  Mr.  S.  F.  Batchelder  of  the  Senior  class  of  Harvard  College, 
1  *'.»:!.  The  chart  from  whjch  Fig.  18  is  reduced  was  constructed  by  Mr.  J.  L.  S.  Connolly. 
of  the  same  class.  See  American  Meteorological  Journal,  vol.  x. 


74 


ELEMENTARY    METEOROLOGY. 


negative  annual  anomaly  is  found  near  the  equator  west  of  South  America, 
more  strongly  marked  than  anywhere  else  in  the  torrid  zone  ;  this  being  the 
result  of  the  long  reach  made  by  the  slender  extremity  of  South  America  into 
the  Antarctic  eddy,  thereby  turning  a  great  volume  of  cold  southern  water 
towards  the  equator.  A  peculiar  effect  of  this  cold  current  and  the  consequent 
anomalous  temperatures  that  it  causes  on  the  equator  is  to  exclude  coral  polyps 
from  the  Galapagos  islands,  while  they  flourish  on  the  shores  of  similar  islands 
further  west  in  the  Pacific,  where  the  ocean  current  has  become  warm  by  longer 
exposure  to  equatorial  sunshine. 

90.  Annual  range  of  temperature.  Several  important  generalizations  are 
found  on  studying  the  distribution  of  large  and  small  annual  ranges  of  tempera- 
ture, as  determined  by  the  difference  between  the  means  of  the  warmest  and 


LINES  OF 

EQUAL  ANNUAL  RANGE 
OF  TEMPEKATUKE 


FIG.  18. 

coldest  months.  Lines  of  equal  annual  range  are  drawn  in  Fig.  18. l  In  the 
first  place,  the  area  of  moderate  annual  range  —  less  than  10°  P. —  extends  nearly 
all  over  the  torrid  zone,  where  the  annual  variation  of  insolation  is  small ;  and 
over  a  large  part  of  the  southern  oceans,  even  to  comparatively  high  latitudes, 
because  the  waters  change  their  temperatures  with  so  great  difficulty ;  but  on 
the  polar  seas,  the  annual  range  is  strong  in  spite  of  their  conservatism. 
because  there  the  variation  of  insolation  is  so  great.  Passing  next  to  areas  of 
the  most  extreme  range  —  over  70°  —  these  will  be  found  only  on  the  large 

1  See  note  to  Figs.  l<>  and  17. 


DISTRIBUTION    OF    ATMOSPHERIC    TEMPERATURES.  75 

laud  areas  far  from  the  equator ;  hence  no  areas  of  extreme  rauge  are  found 
in  the  southern  hemisphere ;  they  a^e  limited  to  the  northern  portions  of  the 
northern  continents,  where  the  strong  variation  of  insolation  readily  causes  a 
strong  change  in  temperature.  Great  range  of  temperature,  therefore,  char- 
acterizes a  continental  climate  in  temperate  latitudes,  while  a  small  range 
characterizes  an  oceanic  climate  in  nearly  all  parts  of  the  world. 

A  peculiarly  unsymmetrical  distribution  of  large  and  small  ranges  is  found 
on  the  eastern  and  western  coasts  of  our  northern  continents.  A  belt  of  small 
range  of  temperature  —  not  over  25°  —  extends  all  along  our  Pacific  'coast,  and 
all  along  the  western  coast  of  Europe  ;  while  the  area  of  strong  although  not 
the  most  excessive  range  —  over  45°  —  extends  along  our  eastern  coast  and 
over  the  eastern  coast  of  Asia.  The  reason  for  this  is  to  be  found  in  the 
combined  action  of  ocean  currents  and  winds,  particularly  in  the  control  of 
the  distribution  of  temperature  by  the  latter.  In  temperate  latitudes,  the 
prevailing  course  of  the  winds  is  almost  from  west  to  east.  The  western  coasts 
of  the  continents  are  therefore  breathed  upon  by  the  winds  coming  from  the 
oceans  ;  these  are  comparatively  mild  in  summer  and  cool  in  winter :  hence 
the  range  of  temperature  that  they  allow  over  the  coastal  land  is  small.  On  the 
eastern  coasts,  the  winds  blow  from  land  to  sea  and  carry  with  them  the 
extreme  changes  from  cold  winters  to  hot  summers.  The  western  coasts  con- 
sequently savor  of  an  oceanic  climate ;  while  the  eastern  coasts  partake  of  a 
continental  climate.  The  eastern  coast  of  Asia  is  somewhat  protected  against 
the  extreme  cold  of  Siberia  by  mountain  ranges  some  distance  inland.  The 
eastern  part  of  the  United  States  has  no  such  barrier  to  keep  back  the  cold 
winds  from  the  far  Northwest ;  hence  the  severity  of  our  winter  cold  waves. 

91.  Polar  temperatures.  The  table  in  Section  27  gave  remarkably  high 
values  for  the  insolation  received  at  the  poles  on  the  solstitial  days  when  the 
sun  rose  to  the  greatest  altitude  over  the  polar  horizons.  If  temperature 
followed  insolation  directly,  we  should  find  the  hottest  days  of  the  world 
and  of  the  year  at  the  south  pole  on  December  21,  and  at  the  north  pole  on 
June  20.  As  a  matter  of  fact,  the  north  and  south  poles  are,  as  well  as  can 
be  inferred,  the  coldest  places  in  their  respective  hemispheres  on  these  days. 
The  reason  for  the  small  rise  of  temperature  around  the  poles  in  spite  of  the 
great  amount  of  insolation  showered  upon  them  is  found,  first,  in  the  necessity 
of  melting  the  ice  and  snow  before  the  land  temperature  can  rise  above  32°; 
second,  in  the  large  water  areas  near  the  poles  :  third,  in  the  brief  duration 
of  strong  polar  insolation. 

Mention  should  be  made  of  the  peculiar  migration  of  what  is  known  as  the 
northern  "  cold  pole  "  or  center  of  lowest  temperature  in  the  northern  hemi- 
sphere in  winter.  In  the  southern  hemisphere,  we  may  reasonably  expect  the 
south  pole  to  be  the  coldest  spot  throughout  the  year ;  for  it  is  within  an  icy 


76  ELEMENTARY   METEOROLOGY. 

plateau,  surrounded  by  wide  oceans.  In  the  northern  hemisphere,  while  it  is 
true  that  for  the  mean  temperature  of  the  year,  the  region  immediately  around 
the  north  pole  seems  to  be  the  coldest  place  in  the  hemisphere,  and  while  the 
polar  area  is  probably  the  coldest  part  of  our  hemisphere  in  summer  also,  this 
does  not  seem  to  be  the  case  in  winter,  as  far  as  the  present  records  of  Arctic 
temperatures  go.  In  that  season,  in  spite  of  the  continuous  darkness  at  the 
pole  for  five  months,  the  temperature  within  the  polar  ocean  is  probably  not 
below  — 42°,  while  in  a  certain  part  of  northeastern  Siberia,  the  mean  tempera- 
ture for  January  is  — 60°.  The  reason  for  this,  and  for  many  other  facts  of 
temperature  distribution,  is  found  in  the  more  rapid  cooling  of  land  than 
of  water,  as  well  as  in  the  circulation  of  the  Arctic  ocean  waters,  to 
which  reference  has  already  been  made.  The  charts  of  our  winter  season 
therefore  represent  a  strongly  marked  cold  pole  in  northeastern  Siberia, 
from  which  the  temperature  rises  in  all  directions,  north,  south,  east,  and 
west. 

It  is  possible,  however,  if  future  exploration  discovers  a  considerable  land 
area  near  the  north  pole,  that  the  temperature  there  may  fall  to  even  a  lower 
degree  for  January  than  is  observed  at  the  Siberian  "  cold  pole."  In  this  case 
we  may  speculate  as  to  the  seat  of  lowest  temperature  in  the  northern  hemi- 
sphere in  summer.  A  polar  land  area  would  reach  a  comparatively  high 
temperature  in  June  and  July,  if  not  too  heavily  clad  with  snow  and  ice  ;  and 
it  might  then  be  even  warmer  than  the  ocean  around  it.  In  that  case,  the 
summer  "  cold  pole  "  would  be  an.  annulus  of  low  temperature  around  a  polar 
oasis  of  somewhat  higher  temperature. 

In  later  chapters,  we  shall  return  again  to  the  question  of  temperature  and 
its  distribution  in  considering  types  of  weather  and  the  climatic -features  -of 
the  world ;  but  before  these  are  taken  up,  the  control  of  the  circulation  of  the 
atmosphere  by  its  differences  of  temperature  and  the  consequences  of  the 
circulation  in  producing  clouds,  storms  and  rain  must  be  examined. 


THE    PRESSURE    AND    CIRCULATION    OF    THE    ATMOSPHERE.  77 

CHAPTER   VI. 

THE  PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE. 
GENERAL  PRINCIPLES. 

92.  The  conditions  of  general  convectional  motion.    -If -the  atmosphere 
were  everywhere  of  uniform  temperature,  it  would  lie  still  on  the  earth's 
surface,  and  there  would  be  no  winds  ;  but  we  have  learned  that  the  tempera 
ture  of  the  atmosphere  is  continually  or  periodically  higher  in  one  region  than 
in  another,  and  that  the  chief  variations  in  the  distribution  of  temperature  are 
systematically  repeated,  year  after  year.     This  prevents  the  stagnation  of  the 
atmosphere  and  ensures  a  systematic  movement  of  air  currents  from  place  to 
place.    The  general  principles  on  which  such  movements  depend  are  extremely 
simple  and  will  now  be  briefly  stated,  in  order  to  prepare  the  way  for  a  better 
understanding  of  the  charts  of  atmospheric  pressure  and  winds,  whicli  soon 
follow. 

Let  it  be  supposed  that  a  certain  part  of  the  atmosphere  is  maintained  at 
a  higher  temperature  than  that  of  its  surroundings.  The  warmed  air  will  be 
expanded ;  its  upper  layers  will  flow  off  to  the  surrounding  regions,  cooling  as 
they  go,  and  the  pressure  at  sea-level  will  thereby  be  decreased  in  the  warm 
region  and  increased  round  about  it.  The  lower  air,  impelled  by  these  differ- 
ences of  pressure,  will  creep  in  beneath  the  warmed  air,  warming  as  it  goes, 
and  thus  a  regular  convectional  circulation  will  be  established  on  a  large 
scale.  As  before,  in  Section  53,  this  may  be  likened  to  the  working  of  a 
clock:  v&  expend  a  certain  amount  of  muscular  energy  in  winding  up  the 
weight  against  gravity,  and  gravity  then  pulls  it  down  again,  driving  the 
wheels  and  the  hands.  The  sun  warms  a  certain  part  of  the  air,  thereby 
causing  it  to  expand  upwards  against  gravity ;  in  other  words,  lifting  the  upper 
layers  by  the  expansion  of  the  lower  ones.  Gravity  then  pulls  the  upper 
layers  down  on  the  surrounding  unexpanded  air,  and  the  differences  of  pressure 
thus  introduced  drive  the  other  members  of  the  circulation. 

93.  Arrangement  of  isobaric  surfaces  in  a  general  convectional  circulation. 
This  problem  deserves  deliberate  illustration.     Let  Fig.  19  represent  a  vertical 
section  of   the  atmosphere,  in  which  the  vertical  scale  is   much  magnified 
compared  to  the  horizontal.     If  the  temperatures  be  uniform  at  every  succes- 
sive level,  although  decreasing  at  the  usual  rate  from  below  upwards,  then  the 
isobaric  surfaces  will  be  level  and  concentric,  as  explained  in  Section  18.     This 
arrangement  is  indicated  by  the  broken  lines,  numbered  at  the  margin  to 


78: 


ELEMENTARY    METEOROLOGY. 


-e  

-fcr- 

Sea 

is 

Level 

30 

FIG.  19. 


indicate  their  respective  pressures.  Let  it  now  be  supposed  that  the  tempera- 
ture of  all  the  central  region  is  raised  by  a  certain  amount.  All  the  air  thus 
warmed  will  expand.  The  column  HA  will  expand  to  height  HB ;  and  hence 
the  isobaric  surface  of  29  inches  will  take  the  position  DBC.  All  the  overlying 

surfaces  will  be  similarly  bowed 
upwards,  the  highest  of  them  being 
arched  by  the  greatest  amount.  Con- 
sider now  the  condition  of  any  two 
volumes  of  air  at  equal  heights,  one 
in  the  warmed  region  at  E9  the 
other  in  the  cooler  space  at  F.  The 
pressure  on  .7?  is  27.10  ;  the  pressure 


on  F  is  26.90.  These  volumes  exert 
an  expansive  pressure  in  all  direc- 
tions equal  to  the  pressure  upon  them.  Hence  the  series  of  similar  volumes 
between  E  and  F  will  not  be  in  equilibrium,  but  will  be  pushed  outward  to  the 
left ;  and  if  this  outward  force  suffices  to  overcome  friction,  movement  in  that 
direction  will  ensue. 

This  condition  of  motion  may  be  illustrated  by  the  principle  of  the  inclined 
plane,  as  follows.  Let  ST,  Fig.  20,  be  a  magnified  part  of  one  of  the  bowed 
isobaric  surfaces  of  Fig.  19.  As  far  as  the  air  above  this  plane  is  concerned, 
all  the  •  lower  air  might  for  the  moment  be  removed  and  replaced  by  any  rigid 
body  having  a  surface,  ST.  A  volume  of  air,  G,  resting  on  the  plane,  is  drawn 
down  by  the  gravitative  force,  Gg.  This  may  be  analyzed  into  two  compo- 
nents, one  of  which,  Gp,  at  right  angles  to  ST,  causes  no  motion,  while  the 
other,  Ga,  parallel  to  the  plane  in  the  direction  of  its  descent,  urges  the  air 
to  move  down  the  slope. 

In  whichever  way  the  problem  is  viewed,  it  is  clear  that  the  upper  air 
above  the  warmed  region  is  impelled  to  move  away  laterally  and  accumulate 
over  the  cooler  surrounding  region.  In 
consequence  of  such  movement,  the  pres- 
sures at  sea-level  will  be  rearranged.  Let 
it  be  supposed  that  the  pressure  at  H  is 
thus  decreased  to  29.50  ;  while  that  on 
the  marginal  area  is  increased  to  30.25. 
What  will  be  the  position  of  the  isobaric 
surface  of  30.00  under  this  new  arrange- 
ment ?  It  could  be  found  beneath  //  by 
descending  a  shaft  about  450  feet  deep,  as  at  Jt  Fig.  21.  It  could  be  found  above 
K  by  rising  about  225  feet  into  the  air.  The  pressure  /f  being  L>(.).r>0,  and  at  K 
being  30.25,  the  pressure  of  30.00  at  sea-level  may  be  expected  at  M,  one  third  of 
the  distance  from  K  to  H.  The  curved  lim-  r,M./!lf'L  '  may  now  be  drawn,  as  in- 


THE    PRESSURE    AND   CIRCULATION    OF    THE   ATM<  )SPH  ERE.  79 

dicating  with  sufficient  accuracy  the  new  position  of  the  isobaric  surface  of  30.00. 
From  this  as  a  base,  the  overlying  surfaces  may  be  constructed  ;  for  the  height 
of  a  column  of  air  between  adjacent  surfaces  corresponding  to  a  barometric 
Mich  under  a  given  pressure  and  at  a  given  temperature,  may  be  taken  froiu 
meteorological  tables.     It  is,  how-  ^  _  _____________________    ^        „ 

t'ver,  manifest  from  inspection  that    rrr^ 

the  distance  between  any  pair  of       N         ______________________      1/2?" 

isobaric  surfaces  must  increase  in    ---- 

passing    from    the    cooler    to   the  t  --------     -----------  -^.^    28 

warmer  region  ;  or,  in  other  words, 

that  the  whole  system  of  rearranged 

isobaric  surfaces  must  diverge  from 

one  another  as  they  enter  the  warm 

region.     Hence  the  central  depres-  FlG-  21- 

sion  seen  in  the  concave  surface  of  30.00  will  gradually  weaken  in  the  higher 

surfaces,  and  if  we  ascend  high  enough  it  must  entirely  disappear  and  be 

replaced  by  a  central  elevation  in  a  series  of  convex  surfaces.     One  of  these  is 

shown  in  the  uppermost  full  line  of  Fig.  21.     Between  this  line  of  26  inches 

pressure  and  the  isobar  of  27  inches,  other  isobaric  surfaces  might  be  drawn 

for  the  fractional  parts  of  the  inch  ;  and  one  of  these  at  the  height  NN1  would 

be  level  ;  this  is  called  the  neutral  plane.     Above  it,  the  air  tends  to  flow 

outward  ;  below  it,  the  air  follows  the  slope  of  the  isobaric  surfaces  and  tends 

to  creep  inward. 

94,   Conditions  of  steady  motion.     The  arrangement  of  the  isobars  thus 
determined  by  the  first  outflow  aloft  suffers  still  further  change  on  account 
of  the  inflow  established  below.     The  strong  difference  between  central  and 
marginal  pressures  that  was  first  assumed  is  diminished,  and  finally  falls  to 
just  that  value  by  which  a  steady  circulation  can  be  maintained,  as  illustrated 
^  ___________________  -^  2^"     in  Fig.  22.    Inasmuch  as  the  resist- 

"***     ***  ~  ances  to  motion  encountered  by  the 

____  ^Y  __  _       _N__*£     air  are  very  small,  the  final  arrange- 


ment of  the  isobars  has  very  faint 
slopes.  The  initial  difference  of 
temperature  between  the  central 
and  marginal  regions  is  lessened 
by  the  interchange  of  air  that  it 
produces  ;  but  as  long  as  any  differ- 
ence of  temperature  is  maintained, 

a  system  of  diverging  isobaric  surfaces  will  be  maintained  also,  with  outward 
slope  above  and  inward  slope  below.  The  action  of  gravity  on  the  inclined 
isobaric  surfaces  will  then  be  entirely  expended  in  overcoming  the  resistances 


80  ELEMENTARY  .METEOROLOGY. 

excited  by  the  motion,  and  not  in  accelerating  the  motion  to  higher  ard  higher 
velocities.  This  is  like  the  case  of  a  train  of  cars  which  an  engine  is  pulling 
with  all  the  force  of  high  pressure  steam,  and  which  nevertheless  does  not 
exceed  a  certain  speed  of  travel  :  the  resistances  excited  by  the  movement  of 
the  train  have  then  risen  to  equality  with  the  pull  of  the  engine,  and  no  higher 
speed  can  be  attained. 

In  the  case  of  a  large  convectional  circulation  in  the  atmosphere,  the 
velocity  of  steady  motion  will  be  greater  if  the  central  region  is  kept  very 
warm.  If  the  central  region  is  maintained  only  a  few  degrees  above  the 
temperature  of  the  surrounding  region,  the  velocity  gained  will  be  moderate. 
If  the  supply  of  heat  for  the  central  region  varies  periodically,  the  differences 
of  pressure  produced  and  the  velocities  maintained  by  them  will  vary  in  the 
same  period,  changing  with  the  rise  and  fall  of  central  temperatures.  It,  the 
central  region  is  cooled  below  the  temperature  of  the  surrounding  region,  it 
will  become  an  area  of  high  pressure,  with  outflowing  surface  winds.  If  the 
central  region  is  alternately  warmer  and  colder  than  the  surrounding  region, 
the  direction  of  the  circulation  will  be  changed  in  corresponding  periods  ;  the 
surface  winds  flowing  inwards  when  high  temperature  and  low  pressure  pre- 
vail, and  outwards  when  the  conditions  are  reversed. 

95.  Barometric  gradients.    The  term,  gradient,  has  already  been  introduced 
in  connection  with  the  vertical  diminution  of  temperature,  to  indicate  a  rate  of 
decrease.     We  now  find  need  of  the  same  term  in  connection  with  the  decrease 
of  barometric  pressure  in  passing  along  a  horizontal  surface,  such  as  sea  level, 
from  the  margin  to  the  center  of  a  warmed  region,  or  from  the  center  to  the 
margin  of  a  cold  region.     If  the  slope  of  the  isobaric  surfaces  is  steep,  the 
horizontal  decrease  of  pressure  will  be  rapid,  and  the  gradient  will  be  strong ; 
if  the  slope  is  gentle,  the  gradient  will  be  weak.     Hence  the  barometric  or 
baric  gradient,  commonly  taken  to  measure  only  a  rate  of  decrease  of  pressure 
along  a  horizontal  surface,  also  indicates  the  amount  of  slope  of  an  isobaric 
surface.     The  direction  of  decrease  or  of  slop.-  is  commonly  stated  with  the 
rate.     The  rate  of  decrease  is  commonly  expressed  in  hnndredths  of  an  inch  of 
pressure  in  a  quarter  of  a  latitude-degree  of  horizontal  distance.1     This  subject 
will  be  met  again  in  Section  113. 

96.  Vertical  components  of  a  convectional  circulation.     The  ascent  and 
descent  of  air  currents  caused  by  local  diurnal  convection  have  been  described 
in  Sections  45  to  54.     The  vertical  movements  of  the  air  may  become  rapid 
under  favorable  conditions,  as  on   warm   Irvrl  plains,  when  dust  whirlwinds 
spring  up  towards  noon.     The  vertical  movement  may  then  greatly  exceed  the 

1  This  way  of  measuring  the  gradient  is  recommended  because  its  numerical  value  is  then 
unchanged  if  expressed  in  millimeters  of  pressure  per  latitudr-di-^n-r  of  distance. 


THE    PRESSURE    AND    CIRCULATION    OF    THE    ATMOSPHERE.  81 

accompanying  horizontal  movements,  both  in  velocity  and  in  distance  traversed ; 
but  in  the  examples  of  larger  convectional  circulations  here  considered,  the 
case  is  quite  different.  The  diagrams  employed  in  the  previous  section  are  so 
greatly  exaggerated  vertically  that  they  give  wrong  ideas  in  this  respect,  unless 
arc  is  taken  to  conceive  of  the  circulation  in  its  true  proportions.  It  must  be 
borne  in  mind  that  all  the  larger  examples  of  convectional  circulation  in  the 
atmosphere  have  much  greater  horizontal  than  vertical  dimensions.  A  vertical 
thickness  of  possibly  twenty  miles  may  be  allowed  for  the  general  circulation 
between  the  equator  and  poles,  and  much  less  than  that  for  the  circulation 
between  the  continents  and  oceans  ;  but  the  horizontal  distances  over  which  the 
circulating  winds  travel  may  be  measured  in  hundreds  or  thousands  of  miles. 
TSot  only  so ;  the  cross-section  of  the  ascending  currents  in  the  warm  or  cold 
central  region  may  have  a  much  greater  area  than  the  cross-section  of  the  inflow- 
ing or  outflowing  winds  ;  hence  the  velocity  of  ascent  or  descent  in  the  central 
area  may  be  small  compared  to  the  velocity  of  inflow  or  outflow  around  it.  It 
follows  from  this  that  the  vertical  components  of  the  larger  atmospheric  con- 
/vectional  motions  are  comparatively  inconspicuous  ;  they  have  low  velocities, 
(and  the  regions  over  which  they  occur  occupy  but  a  small  share  of  the  area 
[swept  over  by  the  whole  circulation  ;  they  may  be  much  confused  by  local 
(convectional  currents,  like  eddies  in  the  general  downstream  flow  of  a  river. 

It  is  desirable  to  gain  a  clear  conception  of  these  relations,  as  well  as  of 
Vthe  process  by  which  the  circulation  of  the  atmosphere  is  kept  up,  in  order  to 
(^avoid  certain  careless  forms  of  statement.  It  is  not  uncommon  to  hear  it 
(said:  "The  air  is  heated  and  rises,  and  the  cold  air  rushes  in  from  either 
!jside  to  fill  the  vacuum  thus  formed."  It  is  better  to  express  the  facts  of  the 
^case  by  saying :  "  As  the  air  is  heated,  it  expands  and  overflows  aloft ;  the 
^colder  air  then  creeps  in  beneath  from  either  side,  warming  as  it  goes,  and 
(raising  the  warmer  air  slowly  above  it."  This  places  the  slow  ascensional 
(movement  in  the  warmed  region  at  its  true  low  value,  and  correctly  suggests 
[that  the  driving  force  of  the  circulation  is  found  in  the  gravitative  pressure  of 
Jthe  colder  air  from  the  sides. 

fc     97.   Application  of  the  general  principles  of  convectional  motion  to  the 
case  of  the  atmosphere.     The  knowledge  gained  in  the  foregoing  chapter 
concerning  the  control,  distribution  and  variation  of  temperature  in  the  atmos- 
phere should  enable  the  student  to  make  correct  application  of  the  principles 
(stated  in  the  preceding  sections.     He  should  expect  to  find  a  belt  of  low 
pressure  around  the  heat  equator,  with  caps  of  high  pressure  over  the  poles  ; 
Cthe  equatorial  belt  of  low  pressure  should  migrate  north  and  south  after  the 
Csun,  and  the   contrasts   between   equatorial   and   polar   pressures  should  be 
("greater  in  the  winter  than  in  the  summer  hemisphere.     The  continents  should 
rhave  lower  pressures  than  the   surrounding  oceans   in  summer,  and  higher 


82  ELEMENTARY    METEOROLOGY. 

pressures  in  winter.  The  regions  of  marked  abnormal  temperatures  for  their 
latitude,  such  as  those  of  abnormal  warmth  in  the  northern  Atlantic  and 
Pacific,  should  have  correspondingly  abnormal  pressures.  The  winds  should 
blow  outward  from  the  regions  of  high  pressure  towards  those  of  low  pressure ; 
hence  there  should  be  a  system  of  winds  blowing  from  either  pole  towards  the 
equator,  but  more  or  less  modified  by  an  indraft  towards  the  continents  in 
their  summer  season  and  an  outflow  in  their  winter.  The  uppermost  currents 
should  move  opposite  to  the  surface  winds.  The  velocity  of  the  winds  should 
be  greater  where  the  barometric  gradients  are  stronger ;  hence  greater  in  the 
winter  hemisphere  as  a  whole.  Along  the  axis  of  the  equatorial  belt  of  low 
pressure,  as  well  as  in  the  centers  of  the  polar  and  continental  areas  of  high 
or  low  pressure,  where  there  are  no  gradients,  there  should  be  no  winds  ;  that 
is,  calms  should  prevail.  With  these  deductions  in  mind,  the  charts  of 
pressure  and  of  the  winds  for  the  year  should  be  examined.  Certain  supple- 
mentary explanations  must  be  introduced  before  all  the  facts  concerning  the 
^pressure  and  circulation  of  the  atmosphere  can  be  understood,  but  no  proper 
beginning  can  be  made  in  this  chapter  without  an  understanding  of  the 
theory  of  convectional  circulation. 

THE  MEASUREMENT  AND  DISTRIBUTION  OF  ATMOSPHERIC  PRESSURE. 

98.  Measurement  of  atmospheric  pressure.     Reference  was  made  in  an 
earlier  chapter  to  the  pressure  exerted  by  the  quiet  atmosphere  upon  the  level 
surface  of  the  ocean.     The  pressure  would  be  uniform  all  over  the  world,  if 
there  were  no  differences  of  temperature  and  no  winds.     We  must  now  investi- 
gate the  means  of   determining  the  actual  pressure  at  any  place,  and  the 
general  distribution  of  pressure  over  the  surface  of  the  earth  under  existing 
conditions.     This  requires,  first,  an  examination  of  the  construction  and  use 
of  barometers,   and,   second,   the  study  of    charts    on   which   the  results  of 
barometric  observations  are  displayed. 

Barometers  are  of  two  kinds.  In  instruments  of  one  kind  the  pressure  of 
the  atmosphere  is  counterbalanced  by  the  weight  of  a  column  of  liquid  of 
known  density  and  measurable  height ;  as  the  liquid  employed  is  usually 
mercury,  these  are  called  mercurial  barometers.  In  instruments  of  another 
kind  the  atmospheric  pressure  is  balanced  by  a  spring  inside  of  a  closed 
metallic  box  from  which  the  air  has  been  exhausted,  so  that  any  change  in 
external  pressure  deforms  the  box  slightly.  These  are  called  aneroid  barom- 
eters, from  being  made  without  employing  a  liquid. 

99.  Mercurial  barometers  are  made  on  the  principle  already  explained  on 
11.     Various  special  devices  are  employed  to  simplify  the  measurement 

of  the  height  to  which  the  mercury  column  is  held  up  in  the  tube  above  the 


THE    PRESSURE   AND    CIRCULATION    OF    THE    ATMOSPHERE. 


88 


level  of  the  surface  of  the  mercury  in  the  dish  or  vessel  on  which  the  air 

presses.      The  Fortin   barometer,  illustrated   in   Fig.   23,  is 

most  commonly  employed  in  the  stations   of    the  Weather 

Bureau.     The  glass  tube  containing  the  mercury  is  enclosed 

in  a  brass  tube,  open  in  the  upper  part  on  two  sides,  so  that 

the  top  of  the  mercury  column  may  be  seen;  it  is  graduated  to 

inches  and  tenths  on  the  edge  of  the  opening  (a).     The  air 

gains  access  to   the  surface   of  the   mercury   in   the  vessel 

below  through  the  fine   crevices  at  the   top   of  the   vessel, 

where  a  glass  ring  (b)  joins  the  base  of  the  brass  tube.     The 

bottom   of  the   vessel   is   a   buckskin   bag,   within   a  brass 

cylinder  (c),  against  which  a  thumb-screw  (d)  presses  from 

below,  so  that  the  height  of  the  mercury  surface  in  the  vessel 

can  be  raised  or  lowered  until  it  just  touches  the  end  of  a 

fine  ivory  pointer  (inside  the  glass  ring,  b,)  which  represents 

the  zero  point  of  the  brass  scale.     When  thus  set,  the  height 

of  the  mercury  in  the  tube  can  be  accurately  read  by  a  vernier 

or  index  (v)  that  slides  in  the  opening  of  the  brass  tube.     A 

good  instrument  of  this  kind  costs  about  thirty  dollars. 

100.  Correction  for  temperature.     Readings  thus  made 
must  be  corrected  for  temperature,  because  of  the  expan- 
sion of  the   mercury  as  well  as  of  the   brass   tube  which 
carries  the  scale.     If  two  barometers  were  under  the  same 
atmospheric   pressure,    one   in   the   cold   outer   air   and   the 
other   in   a  warm  room,  the  latter  would  read  higher  than 
the   former.       To   make   allowance    for   this,    the   tempera- 
ture of  the  barometer   is  determined  by  a  thermometer  (#) 
attached  to  the  tube,  and  all  readings  are  reduced  to  what 
they  would  be  if  the  temperature  of  the  whole  instrument 
were  32°,  by  means  of  corrections  given  in  barometric  tables. 
This  correction  must  be  applied  before  different  readings  are 
compared. 

101.  Correction  for  latitude.     The  force  of  gravity,  by 
which  the  spheroidal  form  of  the  ocean  surface  is  determined, 
is  not  a  constant,  but  varies  from  a  maximum  at  the  poles 
to  a  minimum  at  the  equator,  its  greatest  and  least  values 
being  in  the  proportion  of  193  to  192.     It  follows  from  this 
that  if  there  were  a  uniform  atmospheric  pressure  over  the 
earth,  the  barometric  readings  at  sea-level  would  vary  ;  being 
progressively  greater  towards  the  equator  and  less  towards 


84  ELEMENT  All  Y    METEOKOLOGY. 

the  poles  than  at  latitude  45°,  where  the  average  reading  would  be  found. 
Henee.  in  careful  comparisons  of  mercurial  barometric  observations  at  different 
latitudes,  the  readings  must  be  corrected  by  reducing  them  to  some  standard 
latitude,  as  45°,  by  the  following  table  :  — 

Latitude  ....  1)0°      80        70        60        50       40        30       20        10         0 

Correction  .  .  .  +0".08  +.07  +.00  +.04  +.01  -.01  -.04  -.06  -.07  -.08 

It  appears  from  this  that  a  reading  of  30.00  inches  or  762  mm.  at  the 
equator  corresponds  to  29.92  inches  or  760.00  mm.  at  latitude  45°.  The  varia- 
tions of  atmospheric  pressure  seldom  range  over  an  inch  and  a  half  during  an 
entire  year,  although  a  change  of  half  an  inch  in  a  day  is  not  rare  in  our 
latitudes.  Dealing  with  quantities  of  so  small  a  magnitude,  it  is  essential 
that  a  good  mercurial  barometer  should  read  accurately  to  a  hundredth  of  an 
inch  at  least.  Instruments  of  less  accuracy  are  not  serviceable  in  serious 
study,  although  they  may  be  useful  in  indicating  weather  changes. 

102.  Aneroid  barometers.     Ordinary  variations  in  atmospheric  pressure 
suffice  to  cause  a  small  change  in  the  shape  of  a  spring  within  a  metallic  box 
or  case  torn  which  the  air  has  been  exhausted.     The  changes  may  be  magnified 
by  means  of  a  system  of  levers,  which  finally  turn  a  hand  on  a  dial  to  indicate 
higher  or  lower  atmospheric  pressure ;  the  dial  being  graduated  to  correspond 
to  the  inches  of  the   mercurial  barometer.     The  errors   involved  in  such  u 
mechanism  are  too  great  to  warrant  full  confidence  in  the  readings  of  the  dial. 
The  direct  microscopic  reading  of  an  arm  soldered  to  the  side  of  the  metallic 
box  gives  better  results,  but  is  seldom  employed.     A  good  aneroid  barometer, 
costing  twenty  or  thirty  dollars,  is  of  value  as  a  "  weather  glass,"  if  carefully 
observed ;  but  its  readings  are  by  no  means  so  accurate  as  those  of  a  good 
mercurial  barometer.     Observations  of  an  aneroid  barometer  are  not  acceptable 
in  meteorological  records,  unless  the  error  of  the  aneroid  is  known  by  frequent 
comparisons  with  a  good  mercurial  barometer. 

As  aneroid  barometers  depend  on  the  elasticity  of  a  metallic  spring  and 
not  on  the  force  of  gravity,  their  readings  do  not  need  a  correction  for  latitude. 

103.  Barographs.    The  need  of  continuous  records  of  atmospheric  pressure 
has  led  to  the  invention  of  mercurial  and  aneroid  barographs  of  much  value. 
The  mercurial  barographs  give  records  that  may  be  trusted  to  the  hundredth 
of  ;m  inch,  but  their  cost  is  so  great  that  they  cannot  be  commonly  <  mployed. 
Aneroid  barographs  made  by  Richard   Freres  of  Paris  (Fig.  24),  cost  about 

without  duty,  and  like  the  thermographs  of  the  same  makers  deserve 
mucli  more  general  introduction  than  they  have  yet  gained  in  this  country. 
They  should  be  frequently  tested  by  comparison  with  a  good  mercurial 
barometer  and  a  correct  clock  ;  if  this  is  regularly  done,  their  records  may 


THE   PRESSURE   AND    CIRCULATION    OF    THE   ATMOSPHERE.  85 

be  trusted  within  a  fiftieth  of  an  inch.     Sample  barographic  curves  are  given 
below  in  Fig.  25. 

Among  the  most  interesting  records  obtained  from  barographs  are  those 
marking  the  atmospheric  wave  produced  by  the  explosive  eruption  of  the 
volcano  Krakatoa,  in  the  strait  of  Sunda  between  Java  and  Sumatra,  on 
August  26-27,  1883.  The  chief  explosion  occurred  at  ten  o'clock  in  the 
morning  of  August  27,  local  time  (=  2h  56m,  Greenwich  time)  ;  it  spread 
outward  in  all  directions  with  a  velocity  of  about  700  miles  an  hour,  or  a 
little  less  than 'the  usual  velocity  of  sound.  About  eighteen  hours  were 
required  to  pass  around  the  earth  to  the  antipodal  point,  whence  the  wave 


returned  to  Krakatoa  again.  Three  passages  out  and  back  are  indicated  by 
the  barometric  records  from  various  parts  of  the  world,  the  agitation  of  the 
atmosphere  continuing  over  four,  days  before  it  became  imperceptible  (see 
Section  71). 

104.  Diurnal  variation  of  the  barometer.  When  hourly  observations  of 
the  barometer  are  continued  for  a  considerable  period,  or  when  barograph 
records  are  carefully  examined,  the  mean  values  of  pressure  for  the  several 
hours  of  the  day  may  be  determined,  and  a  double  oscillation  of  diurnal  period 
will  then  be  found.  This  is  most  distinct  in  the  torrid  zone,  where  it  amounts 
to  ten  or  twelve  hundredths  of  an  inch,  having  a  chief  maximum  about  ten 
o'clock  in  the  morning,  a  chief  minimum  about  four  in  the  afternoon,  a 

Mtlary  maximum  about  ten  in  the  evening,  and  a  subordinate  minimum 
about  four  in  the  morning.  The  variation  diminishes  towards  the  poles,  and 


86 


ELEMENTARY   METEOROLOGY. 


is  less  in  winter  than  in  summer.  In  this  country  it  is  seldom  over  a  tenth  of 
an  inch.  At  interior  continental  stations  the  subordinate  oscillation  diminishes 
in  value.  The  curve  for  May  in  Fig.  25  is  from  a  record  at  Harvard  College, 
for  a  spell  of  fair  spring  weather,  May  17  to  25,  1887,  in  which  the  diurnal 
variation  is  faintly  shown  ;  the  maxima  by  +,  the  minima  by  0. 


Noon 

Noon                         Noon                          Noon                          Noon 

30.0 
29.5 

J—  ffE?*5 

te- 

tei 

s 

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+ 

1 

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L^ 

4  

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\^r 

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. 

O 

1    +^ 

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o 

Jlciy     o 

fur 

«   o 

\ 

^ 

x 

\ 

^ 

| 

^ 

^/ 

V 

r^^- 

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x^ 

1887 

.Feb.  23 

Feb.Zt                         Feb.Vb                           Feb.26                          *eoxi 
>ay.l»                        May  20                          3foj/21                         MayM 

FIG.  25. 


The  cause  of  the  diurnal  variation  of  pressure  is  undoubtedly  to  be  found 
in  the  diurnal  variation  of  temperature,  but  the  operation  of  the  cause  in 
producing  the  effect  is  not  well  understood. 

105.  Irregular  fluctuations  of  the  barometer,  having  a  period  of  one,  two 
or  three  days,  are  caused  by  the  alternate  passage  of  areas  of  stormy  and  fair 
weather.  Such  changes  are  comparatively  rare  in  the  torrid  zone  ;  but  they 
occur  every  three  or  four  days  in  the  greater  part  of  the  temperate  zones,  and 
in  our  winter  season  these  fluctuations  become  so  strong  that  the  diurnal 
variation  is  hardly  perceptible  ;  as  in  the  February  curve  in  Fig.  25,  copied 
from  a  barograph  record  at  Harvard  College  for  February  22  to  28,  1887, 
when  two  active  stormy  areas  passed  by,  causing  rapid  weather  changes. 
Fluctuations  of  this  kind  are  further  considered  and  illustrated  in  the  chapters 
on  storms  and  011  weather.  Much  longer  fluctuations  have  also  been  detected, 
covering  several  weeks  or  a  month  ;  these  are  generally  called  surges,  but  they 
have  been  little  studied  and  their  cause  is  not  understood. 


106.  Barometer  observations.  Observations  taken  at  7  A.M.,  2  and  9  P.M. 
will  give  a  mean  diurnal  value  of  pressure  practically  free  from  the  effects 
of  the  regular  diurnal  variation ;  but  as  the  irregular  fluctuations  of  pressure 
in  our  latitude  are  much  greater  than  the  regular  variations,  observations  thus 
taken  serve  only  to  give  the  mean  monthly  pressure  ;  and  from  these  the  mean 
annual  pressure.  Other  values  desired  in  discussing  barometric  observations 
are:  the  diurnal  variation,  determined  from  barographic  records  or  from 
hourly  observations  ;  the  mean  monthly  maximum  and  minimum,  from  which 
the  mean  monthly  range  is  determined  ;  the  monthly  extremes;  the  frequency 
of  the  irregular  fluctuations  and  their  average  period  and  value. 


THE    PRESSURE    AND    CIRCULATION    OF    THE    ATMOSPHERE. 

107.  Comparison  of  observations:  reduction  to  sea-level.      Barometric 
observations  made  at  different  heights  above  sea-level  may  be  compared  by 
their  departures  from  their  local  mean  annual  or  normal  values ;  this  being 
the  method  adopted  in  the  early  part  of  this  century.     A  more  satisfactory 
means  of  comparison  is  found  in  reducing  all  the  observations  to  values  that 
would  have  been  obtained  if  they  had  been  made  at  the  same  altitude  ;  for 
example,  at  sea-level.     This  is  done  by  adding  to  each  reading  (corrected  for 
temperature)  a  supplementary  pressure  to  make  up  for  the  imaginary  column 
of  air  that  may  be  conceived  to  reach  downward  from  the  station  of  observation 
to  sea-level.      The  value  of  this  supplementary  pressure  depends  chiefly  on 
three  factors  :  first,  the  altitude  of  the  station ;  second,  the  pressure  at  the 
station,  for  if  the  observed  pressure  be  high,  the  imaginary  column  would 
contain  air  of  greater  density  than  usual ;  third,  the  temperature  of  the  air,  for 
if  the  temperature  at  the  time  of  observation^  higher  than  usual,  the  air  of 
the  imaginary  column  would  be  expanded  to  a  comparatively  low  density. 
Tables  for  the  reduction  of  barometric  observations  to  sea-level,  the  altitude 
of  the  station  being  given,  are  furnished  in  the  Instructions  to  Voluntary 
Observers,  published  by  the  Weather  Bureau. 

In  preparing  single  barometric  records  for  publication,  all  the  data  should 
be  corrected  for  temperature  ;  but  it  is  best  that  they  should  not  be  corrected 
for  altitude  above  sea-level.  The  corrections  for  reduction  of  the  monthly 
means  to  sea-level  should,  however,  be  determined  and  published  with  the 
means  themselves. 

It  must  be  remembered  that  the  pressure  indicated  by  the  barometer 
corresponds  to  the  weight  of  the  atmosphere  only  when  the  air  is  calm.  When 
it  is  moving,  particularly  when  its  velocity  is  high  and  variable,  or  when  its 
temperature  is  rapidly  changing,  the  atmospheric  pressure  on  the  barometer 
may  be  greater  or  less  than  the  weight  of  the  air ;  but  the  difference  must  be 
small  in  all  cases ;  in  thunderstorms,  it  may  reach  l-20th  inch  (Section  254). 

108.  Barometric  determination  of  altitudes.     As  the  rate  of  the  decrease 
of  atmospheric  pressure  upwards  is  known,  it  follows  that  the  barometer  may' 
be  used  in  the  determination  of  the  altitude  of  a  station  above  sea-level.    This 
is  commonly  done  in  exploring  expeditions  and  in  preliminary  surveys,  the 
observations  being  reduced  by  specially  prepared  tables,  such  as  those  published 
by  the  Smithsonian  Institution  at  Washington.    The  determination  of  altitudes 
used  in  reducing  barometric  observations  to  sea-level  should,  however,  be  made 
by  careful  levelling. 

A  convenient  rule  for  finding  the  difference  of  level  between  two  places  by 
means  of  barometric  observations  is  as  follows :  The  difference  of  level  in 
feet  is  equal  to  the  difference  of  pressures  in  inches  divided  by  their  sum  and 
multiplied  by  the  number  55,761,  when  the  mean  of  the  air  temperatures 


88  ELEMENTARY  METEOROLOGY. 

at  the  two  places  is  60°=  If  the  mean  temperature  is  above  60°,  the 
multiplier  must  be  increased  by  117  for  every  degree  by  which  the  mean 
exceeds  60°;  if  less  than  60°,  the  multiplier  must  be  decreased  in  the  same 
way.  For  example,  if  the  lower  station  has  a  pressure  of  30  ".00  and  a 
temperature  of  62°,  and  the  upper  station  has  29".00  and  58°  respectively,  the 
difference  of  level  between  the  two  will  be 

55,761  =  945  feet. 

If  the  lower  values  are  30".15  and  65°;  while  the  upper  values  are  28".67 
and  59°,  then  the  formula  becomes 


X  [55,761  +  (2  X  117)]  -  1409  feet. 

109-  Barometric  charts.  Charts  showing  the  distribution  of  atmospheric 
pressure  are  prepared  in  much  the  same  manner  as  those  already  described  for 
temperature.  When  observations  have  been  continued  for  a  number  of  years, 
the  corrected  mean  annual  and  monthly  values  are  reduced  to  sea-level  and 
charted  upon  a  map  of  the  world  ;  lineg_of  equal  preaaurg_rna.y  then  be  drawn, 
Jbhese  bein^called  isobaric  lines,  or  more  briefly,  isob&rs.  The  charts  prepared 
by  Buchan  or  by  Hann  are  the  most  recent  and  complete  (Section  80).  Charts 
VI,  VII,  and  VIII  are  reduced  from  those  prepared  by  Buchan.  The  general 
distribution  of  pressure  for  the  year  and  for  January  and  July  may  now  be 
considered. 

110.  Isobars  for  the  year.  The  annual  isobars  on  Chart  IV  show  a  belt 
of  slightly  diminished  pressure  running  nearly  around  the  equator  ;  on  either 
side  there  are  belts  of  higher  pressure,  somewhat  irregular  in  shape,  with  their 
middle  lines  about  latitude  35°  north  and  30°  south.  These  high  pressure 
belts  may  be  called  the  meteorological  tropics.1  The  pressure  then  diminishes 
towards  cither  ]>ole,  although  in  the  northern  hemisphere  this  diminution  is 
much  less  marked  and  more  irregular  than  in  the  southern  ;  the  lowest  northern 
pressures  being  in  the  North  Atlantic  and  North  Paeih'e,  oceans.  The  differences 
of  pressure  thus  found  among  the  mean  annual  values  are  truly  very  small, 
their  range  from  the  highest  in  the  North  Pacific  to  the  lowest  in  the  far 
Antarctic  ocean  being  only  a  little  over  an  inch  ;  but  they  are  of  great  signifi 
cance,  as  will  be  seen  when  the  winds  of  the  world  are  examined. 

1  The  tropics  are,  etymologically,  places  of  luniiim  :  lit  nee  the  Tropics  of  Cancer  and 
Capricorn,  where  the  sun  stops  ami  turns  in  its  annual  mim-ation  mirth  and  south.  The  use 
of  the  term  in  the  text  lien-  will  be  found  later  on  to  mark  a  limitation  or  turning  in  the 
course  of  the  winds  as  well  as  in  the  direction  of  tin-  -ladicnts,  <>f  -iv;it  climatic  importance  ; 
irn-ater.  indeed,  than  that  determined  by  the  zenith  altitude  of  tin-  sun.  Common  usa.irc  often 
ron  founds  the  geographical  tropics  with  the  torrid  zone  which  they  bound.  See  footnote  to 
Section  217. 


THE    PRESSURE    AND    CIRCULATION    OF    THE    ATMOSPHERE. 


89 


FIG.  26. 


111.  Vertical  section  of  the  atmosphere  along  a  meridian.  If  a  vertical 
section  of  the  atmosphere  be  drawn  from  pole  to  pole,  it  will  be  found  to  offer 
certain  instructive  contrasts  to  the  condition  described  in  Chapter  II,  where 
the  isobaric  surfaces  of  the  atmosphere  were  imagined  to  lie  level  and  essen- 
tially parallel  and  concentric  under  the  action  of  gravity  alone.  Fig.  26 
represents  the  desired  section, 
the  meridian  line  at  sea-level 
being  drawn  as  a  straight  line 
in  order  more  easily  to  repre- 
sent the  attitude  of  the  iso- 
baric surfaces  with  respect  to 
it.  The  vertical  scale  is  greatly 
exaggerated.  The  poles  are 
at  N  and  S,  the  equator  is 
uassed  at  Q.  The  pressures 
at  sea-level  are  taken  from 
characteristic  values  on  the 
cli art  of  pressures  for  the 

year.  The  position  of  the  isobaric  surface  of  30.00  is  determined  in  the  way 
employed  in  Section  93  ;  here  being  seen  as  a  line,  it  takes  the  shape  ABCDEF. 
Other  isobaric  surfaces  may  then  be  added  at  greater  heights,  remembering 
that  the  lines  by  which  they  are  represented  must  diverge  in  passing  from  the 
cold  polar  regions  towards  the  warm  torrid  zone.  The  isobaric  surfaces  of 
oO.OO  and  29.90  would  be  98  feet  apart  at  the  equator,  and  82  at  the  pole :  the 
surfaces  of  24.00  and  24.10  would  be  separated  by  about  116  and  96  feet, 
respectively.  Consequently,  the  irregular  warping  of  the  lower  isobaric 
surfaces  is  gradually  changed  to  a  system  of  regularly  convex  isobaric  surfaces 
in  the  upper  atmosphere.  The  equatorial  belt  of  low  pressure  at  sea-level  has 
entirely  disappeared  at  a  height  of  about  12,000  feet ;  there  is  at  that  height 
and  at  all  greater  heights,  a  continuous  poleward  slope  of  the  isobaric  surfaces, 
of  faint  inclination  in  the  torrid  zone,  but  becoming  much  steeper  in  higher 
latitudes  ;  the  greatest  inclination  being  in  the  southern  hemisphere. 

The  arrangement  of  isobaric  surfaces  as  thus  determined  is  a  matter  of  the 
greatest  moment  in  considering  the  circulation  of  the  atmosphere.  Recalling 
what  has  been  said  about  gradients,  it  is  manifest  that'  there  must  be  a  strong 
^ravitative  acceleration  towards  the  pole  in  the  upper  atmosphere,  and  any 
theory  of  atmospheric  circulation  that  does  not  take  this  fully  into  account  is 
faulty. 

112.  Meaning  of  isobaric  lines.  The  method  of  drawing  isobaric  lines 
on  a  chart  has  been  briefly  mentioned  in  Section  109 ;  but  it  should  now  be 
perceived  that  every  isobaricJLine^  on  the  charts  represents  the_iufcersection 


90 


ELEMENTARY    METEOROLOGY. 


of  some  isobaric  surface  with  the  imaginary  sea-level  surface  to  which  all 
barometric  observations  are  reduced.  Under  the  condition  of  uniformly 
distributed  pressure,  assumed  in  Section  18,  there  could  be  no  isobaric  lines, 
for  the  pressure  at  sea-level  would  everywhere  be  about  30.00  ;  but  as  a  matter 
of  fact,  the  atmospheric  pressure  varies  over  the  world  ;  the  isobaric  surfaces 
are  not  level  but  are  warped,  as  is  shown  in  Fig.  26  ;  and  hence  isobaric  lines 
may  be  taken  to  indicate  their  intersections  with  sea-level.  Given  the  isobaric 
lines  on  the  chart,  the  isobaric  surfaces  which  they  stand  for  may  be 
reconstructed.  Any  chart  on  which  isobaric  lines  are  drawn  should  be  inter- 
preted according  to  this  suggestion. 

113.  Interpretation  of  gradients.  Still  another  use  may  be  made  of 
Fig.  26.  Suppose  the  section  is  drawn  north  and  south  through  the  middle 
of  the  Pacific,  where  the  pressure  at  the  equator  is  about  29.80,  and  where  the 
isobar  of  29.90  lies  at  latitude  10°  N.  Let  this  part  of  the  section  be  drawn  to 
a  larger  scale  in  Fig.  27,  in  which  isobaric  surfaces  are  represented  for  every 


FIG.  27. 

two  hundredths  of  an  inch.  In  this  region  it  would  be  said :  the  barometric 
gradient  is  gentle,  to  the  south.  We  may  now  proceed  to  calculate  the  value 
of  the  gradient  in  more  definite  terms.  The  action  of  gravity  on  an  inclined 
isobaric  surface,  as  explained  in  Section  93,  must  be  recalled.  By  what  small 
share  of  gravity  will  the  air  be  urged  to  move  equatorwards  at  the  point  A, 
where  the  isobaric  surface  of  29.90  intersects  sea  level  ?  The  dimensions  of 
the  triangle,  ABC,  may  first  be  determined.  The  base,  Afi,  is  the  distance 
along  the  meridian  corresponding  to  an  increase  of  0.02  in  pressure  ;  and 
according  to  the  chart  of  isobars  for  the  year  this  is  about  two  latitude  degrees, 
or  140  miles,  in  the  region  under  discussion.  The  height,  BC,  may  be  taken 
from  the  table  given  below,  as  equivalent  to  the  height  of  a  column  of  air 
under  a  pressure  of  29.90  and  at  a  temperature  of  78°,  and  corresponding  to 
two  hundredths  of  an  inch  of  barometric  pressure.  This  is  19.4  feet.  It  is 
no\v  desired  to  find  the  ratio  of  Ga  to  Gg.  This  may  be  done  by  the  following 
proportion :  — 

Ga:  Gg  =  BC  :  CA. 

JiA  may  be  substituted  for  CA  without  significant  error,  and  we  have  :  The 
effective  component  of  gravity  at  A  is  to  the  entire  force  of  gravity  as  19.4  is 
t<»  140  X  5280.  The  effective  component  is  therefore  only  .000025  of  gravity. 


Uf/K 

]m 


THE    PRESSURE   AND   CIRCULATION    OF   THE    ATMOSPHERE. 


91 


It  follows  therefore  that  this  extremely  small  force  is  all  that  can  be 
called  on  to  move  the  air  in  the  region  referred  to.  The  gradients  in  other 
regions  may  be  somewhat  weaker  or  stronger,  but  it  will  be  found  that  in  al) 
cases  only  a  small  share  of  gravity  is  at  work  to  maintain  the  circulation  of 
the  atmosphere. 

HEIGHT  OF  A  COLUMN  OF  AIR  EQUAL  TO  ^  INCH  IN  THE  BAROMETER. 

(Arranged  from  Hazen's  Tables.) 


PRESSURE. 

0° 

10° 

20° 

30° 

40° 

50° 

60° 

70° 

80° 

90° 

100° 

22"  .  .  . 

111 

114 

116 

119 

122 

124 

127 

130 

132 

135 

138 

24   ... 

101 

104 

106 

109 

111 

114 

116 

119 

121 

124 

126 

26   ... 

94 

96 

98 

101 

103 

105 

107 

110 

112 

114 

116 

28   ... 

87 

89 

91 

93 

95 

98 

100 

102 

104 

106 

108 

29   ... 

84 

86 

88 

90 

92 

94 

96 

98 

100 

102 

104 

29.5  .  .  . 

83 

85 

87 

89 

91 

93 

95 

97 

99 

101 

103 

30.0  .  .  . 

81 

83 

85 

87 

89 

91 

93 

95 

97 

99 

101 

30.5  .  .  . 

80 

82 

84 

86 

88 

90 

92 

94 

96 

98 

100 

If  these  paragraphs  have  been  appreciated,  it  is  not  top  much  to  say  that 
the  chart  of  annual  isobars  will  have  by  their  assistance  gained  an  entirely 
new  meaning.  The  chart  now  represents  not  only  a  number  of  lines  along 
which  the  pressure  of  the  atmosphere  at  sea-level  is  equal ;  it  represents  the 
linear  intersections  of  the  sea-level  surface  with  a  system  of  warped  isobaric 
surfaces,  inclined  one  way  or  another  at  various  angles.  The  gradient,  which 
before  represented  only  the  direction  and  rate  of  decrease  of  pressure,  or  the 
inclination  of  the  isobaric  surfaces,  is  now  given  a  definite  value  as  a  part  of 
gravitative  force.  Where  the  adjacent  isobaric  lines  are  closer  together  on 
the  chart,  there  the  isobaric  surfaces  must  be  more  steeply  tilted,  and  hence 
there  the  winds  may  be  expected 'to  blow  faster;  where  the  lines  are  further 
apart,  the  inclination  must  be  faint  and  the  winds  should  be  slow.  Along  the 
trough  of  the  equatorial  belt  of  low  pressure,  or  along  the  axis  of  either 
tropical  belt  of  high  pressure,  there  must  be  strips  of  surface  having  practically 
uniform  pressure  ;  here  the  isobaric  surfaces  must  be  parallel  to  the  surface  of 
the  sea ;  here  the  gradient  must  be  zero  ;  here  calms  should  prevail,  interrupted 
by  light  convectional  breezes,  and  this  we  shall  soon  find  to  be  the  fact. 

114.  Isobars  for  January  and  July.  Chart  V,  giving  the  mean  pres- 
sures for  January,  differs  from  that  of  the  year  in  several  suggestive  ways. 
The  axis  of  the  equatorial  belt  of  low  pressure  —  the  barometric  equator  — 
is  found  to  have  shifted  somewhat  to  the  south  of  its  average  annual  position ; 
the  northern  tropical  belt  of  high  pressure  has  greatly  broadened  over  the 
lands,  where  its  pressure  has  increased,  particularly  over  the  greatest  of  all 


92  ELEMENTARY  METEOROLOGY. 

land  areas,  the  Eur-Asian  continent ; l  the  north  polar  pressure  is  somewhat 
higher  than  it  was  for  the  year,  but  the  low  pressure  areas  over  the  northern 
Atlantic  and  Pacific  have  become  more  strongly  marked  than  before',  and 
hence  the  general  extra-tropical  gradients  of  the  northern  hemisphere  are 
stronger  at  this  season  than  for  the  annual  mean. 

Passing  now  to  the  southern  hemisphere,  the  tropical  belt  of  high  pressure 
is  interrupted  over  the  lands,  and  its  average  pressure  is  a  little  less  than  it 
was  in  the  annual  mean ;  the  far  southern  latitudes  show  no  significant 
change,  observations  not  extending  beyond  latitude  70°,  but  the  poleward 
gradients  there  are  slightly  weakened  by  the  small  decrease  of  pressure  in  the 
tropical  belt. 

In  Chart  VI,  for  July,  the  barometric  equator  has  shifted  to  the  north, 
and  the  shift  is  so  strong  over  the  lands  that  the  northern  belt  of  high 
pressure  is  destroyed,  except  on  the  ocean,  where  its  remnants  are  correspond- 
ingly increased;  they  appear  as  two  oval  areas  of  distinct  high  pressure, 
which  lie  farther  north  than  the  axis  of  the  belt  in  January  ;  the  north  polar 
pressure  is  lower  than  in  the  chart  for  January,  but  the  gradients  around  it 
are  on  the  whole  weaker. 

In  the  southern  hemisphere  the  high  pressure  belt  is  almost  continuous 
around  the  world  over  land  and  sea,  and  its  axis  is  farther  north  than  in 
January;  the  polar  pressures  show  no  significant  change  as  fur  as  observed, 
but  the  poleward  gradients  are  slightly  stronger  than  before  on  account  of  a 
slight  increase  in  the  pressures  of  the  tropical  belt. 

It  has  been  calculated  that  the  mean  pressure  over  the  whole  earth  for  the 
year  is  29.89  inches  or  759.20  mm.  The  mean  pressure  for  the  northern 
hemisphere  for  January  is  29.99  ;  for  July.  29.87  :  and  for  the  southern 
hemisphere,  29.79  and  29.91.  It  appears  from  this  that  there  is  a  much 
greater  difference  between*  the  quantity  of  air  over  the  two  hemispheres  in 
January  than  in  July  ;  and  that  the  change  is  due  to  the  shifting  of  an 
amount  of  air  corresponding  to  a  pressure  of  0".12  over  a  hemisphere,  or  a 
weight  of  about  32,000,000  tons  of  air,  from  one  side  of  the  equator  to  the 
other  every  half  year.  The  relation  of  this  semi-annual  variation  to  the 
corresponding  variation  of  temperature  already  considered  is  obvious. 

1  It  should  be  understood  that  the  charted  increase  of  pressure  in  winter  over  continental 
plateaus  is  not  an  actual  increase  shown  by  direct  observation,  but  only  a  relative  im n  n  i 
^een  after  reduction  to  sea-level.  The  actual  pressure  DM  liii^h  plateaus  is  less  in  winter  lh;  M 
in  summer.  The  colder  atmosphere  in  winter  is  compressed  to  lower  levels,  and  haves  1<  a 
air  above  the  elevated  plateau  surface  ;  but  in  the  summer  the  expanded  air  from  other  parts 
nf  the  world  flows  on  the  plateaus,  and  their  pnouie  is  increased.  Yet  if  a  comparison  is 
made  between  the  observed  pressure  on  a  plateau  in  winter  and  the  calculated  pressure  at. 
:ne  altitude  over  the  adjoining  ocean,  the  former  will  be  found  ureaier  thrui  the  latter. 
It  is  this  kind  of  comparison  that  is  iriven  on  the  isoharic  charts,  where  all  observations  aro 
reduc.-d  to  the  same  level. 


immm 


THE    PRESSURE    AND    CIRCULATION    <>F    TliK    ATMOSPHERE.  93 

115.  Suggested  explanation  for  the  distribution  of  pressure.     If  it  were 
not  for  the  low  pressures  at  the  poles,  one  might  say  at  once  that  the  distribu- 
tion of  pressure  is  determined  by  the  distribution  of  temperature.     Where 
high  temperature  prevails,  the  air  would  expand ;  its  upper  layers  would  flow 
off,  leaving  low  pressure,  and  accumulating  in  regions  of  prevailingly  low 
temperature.     Areas  of  high  and  low  pressure  should  therefore  be  expected 
in   areas  of  low  and  high   temperature.     This   is  indeed  true  to  a  certain 
extent :  there  is  a  belt  of  low  pressure  around  the  heat  equator  ;  and  this  low 
pressure  belt  migrates  north  and  south  with,  or  a  little  after,  the  heat  equator. 
The  continents  have  low  pressure  in  summer  and  high  pressure  in  winter,  as 
their  thermal  relations  to  the  surrounding  oceans  would  have  led  us  to  expect. 
The  areas  of  abnormally  high  winter  temperature  far  north  on  the  Atlantic 
and  Pacific  oceans  have  distinctly  low  pressure  at  the  same  time.     But  no 
cause  is  apparent  for  the  tropical  belts  of  high  pressure ;  and  at  the  poles, 
where  the  low  temperature  of  winter  in  particular  would  lead  us  to  look  for 
very  high  pressures,  the  facts  contradict  our  expectation  in  the  most  emphatic 
manner.     The  polar  regions  have  relatively  low  pressure  the  year  round :  the 
Avhole  south  frigid  zone  has  lower  mean  pressures  than  occur  anywhere  else  in 
the  world. 

Before  attempting  further  search  for  an  explanation  of  the  distribution  of 
pressures,  the  general  circulation  of  the  winds  must  be  examined. 

OBSERVATION  AND  DISTRIBUTION  OF  THE  WINDS. 

116.  Winds.     Air  moving  near  the  surface  of  the  earth  and  in  a  nearly 
horizontal  direction  is  called  wind.     Other  motions  may  be  called  air  currents  ; 
but  they  also  are  often  called  winds  in  a  general  way,  as  the  "upper  winds." 

Observations  of  the  wind  should  include  its  direction  and  its  force  or 
velocity.  The  direction  is  determined  by  a  wind  vane,  moving  freely  on  a 
vertical  axis  on  some  elevated  spire  or  pole.  It  is  always  to  be  recorded  as 
the  point  of  the  horizon  from  which  the  wind  comes.  The  arms,  bearing  the 
letters,  N,  E,  S,  W,  by  which  the  vane  is  read,  should  be  carefully  set  to  the 
four  cardinal  points,  allowance  being  made  for  the  local  variation  of  the  mag- 
netic needle  from  the  true  north.  Intermediate  points  should  be  recorded  as 
NW,  EXE,  etc.  The  direction  from  which  the  wind  comes  is  called  wind- 
ward ;  that  to  which  it  goes,  leeward.  A  change  in  the  direction  of  the  wind 
is  called  veering  when  it  progresses  from  left  to  right ;  and  backing  when  the 
shift  is  the  other  way.  The  amount  of  change  is  often  expressed  in  points,  a 
nautical  term  meaning  an  eighth  of  a  quadrant,  or  11£  degrees. 

117.  The  anemoscope  gives  an  automatic  record  of  the  direction  of  the 
wind.     The  wind  vane  turns  a  vertical  rod  that  reaches  down  into  a  room 


ELEMENTARY   METEOROLOGY. 

below :  a  cylinder  is  attached  to  the  lower  end  of  the  rod  and  turns  with  it :  a 
pen  presses  lightly  on  a  paper  wrapped  around  the  cylinder :  the  pen  is  carried 
downward  at  a  slow  and  regular  rate  by  clock-work,  so  as  to  descend  through 
the  length  of  the  cylinder  in  a  day. 

The  vertical  component  of  the  wind's  motions  is  not  detected  by  ordinary 
anemoscopes.  An  anemoscope  may  be  specially  arranged  for  this  purpose, 
having  a  vane  that  moves  on  a  horizontal  axis,  and  which  is  always  pointed 
towards  the  wind  by  a  larger  vane  moving  on  a  vertical  axis  ;  but  this  is 
seldom  used. 


118.  Force  and  velocity  of  the  wind.  The  force  of  the  wind  may  be 
estimated  or  measured.  The  following  scale  is  recommended  in  rating 
different  velocities. 

TABLE  —  Scale,  velocity  and  pressure  of  winds.1 


AVERAGE  VELOCITIES. 


AVERAGE  PRESSURES. 


SCALE.         TERMS.                        Miles  per  hour. 

Meters  per          Pounds  per  square 
second.                         foot. 

Kilograms  per 
square  meter. 

0 

Calm 

0 

0 

0 

0 

1 

Very  light  breeze 

2 

1 

0.03 

0.15 

2 

Gentle  breeze 

7     or  less 

3  or  less 

0.23  or  less 

1.13 

or  less 

3 

Fresh  breeze 

11 

5 

0.64 

3.15 

4 

Strong  wind 

18     or  more 

8  or  more 

1.62  or  more 

7.97 

or  more 

5 

High  wind 

27 

12 

3.64 

17.9 

6 

Gale 

36 

16 

6.48 

31.9 

i 

Strong  gale 

45 

20 

10.12 

49.8 

8 

Violent  gale 

58 

26 

17.12 

84.2 

!i 

Hurricane 

76 

34 

29.26 

143.9 

10 

Most  violent  hurricane 

95 

42 

45.12 

222.0 

A  scale  of  six  numbers  is  often  used,  its  terms  being  light  wind,  moderate 
wind,  strong  wind,  fresh  gale,  whole  gale,  hurricane.  It  is  generally  the  case 
that  the  higher  numbers  of  these  scales  are  too  frequently  employed. 

119.  Anemometers.  It  is  apparent  that  estimates  of  the  velocity  or  force 
of  the  wind  must  be  very  faulty  ;  and  that  instrumental  records  are  much  to  be 
preferred.  A  simple  indication  of  the  pressure  of  the  wind,  from  which  the 
velocity  may  be  obtained  by  the  above  table,  is  obtained  by  means  of  Lind's 
wind-gauge  :  this  consists  of  a  tube.  bent  in  tin-  form  <>!'  U,  containing  a  liquid 
in  its  lower  curve.  One  end  of  the  tube,  bent  hori/outallv,  is  always  directed 
to  the  wind  by  a  vane.  The  pressure  of  tin-  wind  on  the  liquid  then  raises 
the  surface  of  the  liquid  in  the  t'urtlier  ana  of  the  tube,  where  its  height  may  be 

1  The  indefinite  values  of  Nos.  2  and  I  result  fnm;  the  attempt  to  express  the  Smithsonian 
or  Voluntary  Observer  and  tin-  Internal  i.>nal  Bulletin  scale  in  a  sin-le  table. 


THE   PRESSURE    AND    CIRCULATION    OF    THE    ATMOSPHERE. 


95 


read  on  a  scale.  This  instrument  has  the  value  of  being  at  least  consistent  in 
different  places  and  at  different  times  ;  while  estimates  of  wind  force  by 
different  observers  cannot  be  closely  comparable. 

Another  simple  instrument  for  determining  the  force  of  the  wind  consists  of 
a  square  board  or  sheet  of  metal,  hinged  along  its  upper  edge,  and  always 
turned  to  face  the  wind  by  means  of  a  vane.  The  deflection  of  the  square 
from  a  vertical  position  gives  a  measure  of  the  violence  of  the  wind.  The 
maximum  deflection  may  easily  be  automatically  registered,  thus  giving 
indication  of  the  highest  force  reached  by  the  wind  since  the  last  record. 
Sometimes  the  board  is  fixed  in  a  vertical  plane,  but  is  moved  by  the  wind 
against  a  spring ;  this  may  also  register  its  maximum  record. 

The  instruments  just  described  can  hardly  be  trusted  to  give  good  measures 
of  the  velocity  of  the  wind.  For  this  purpose,  more  accurate  methods 
must  be  adopted,  as  in  the  Robinson  anemometer,  Fig.  28.  Four  arms  are 
fixed  at  right  angles,  carrying  hemispherical  cups  at  the  ends  and  rotating  on  a 
vertical  axis.  It  is  found  , 

by  experiment  that  the 
wind  moves  with  about 
two  and  a  third  or  two 
and  a  half  times  the  ve- 
locity of  the  cups.  The 
vertical  axis  may  be  con- 
nected with  a  train  of  cog 
wheels,  so  arranged  as  to 
count  the  rotations  of  the 
arms  and  register  the  num- 
ber of  miles  run  by  the 
wind  on  a  dial  that  may 
be  read  at  certain  hours ; 
or  it  may  make  a  continu- 
ous record  on  a  cylinder 
rotating  by  clock  work,  on 
which  a  pen  marks  the 
successive  miles  of  wind, 
p-- no  rally  by  means  of 
some  electrical  device,  or 
in  some  other  manner,  and 
the  instrument  is  then 
railed  an  anemograph. 

When  the  wind  is  strong,  the  momentum  of  the  whirling  cups  carries  on  their 
movement  through  temporary  lulls  or  slatches  of  the  wind,  and  hence  at  such 
times  the  recorded  number  of  miles  traveled  is  too  great. 


FIG.  28. 


96  ELEMENTARY  METEOROLOGY. 

The  velocity  of  the  wind  in  miles  per  hour  may  be  reduced  to  the  velocity 
in  meters  per  second  (usually  employed  in  Europe)  by  multiplying  by  0.447  ; 
meters  per  second  may  be  reduced  to  miles  per  hour  by  multiplying  by  2.1^7 
or  about  2£. 

An  anemograph  for  vertical  currents  has  sometimes  been  employed,  usin^ 
a  helical  fan  on  a  vertical  axis  to  measure  the  upward  or  downward  movement 
of  the  air.  The  motion  thus  determined  is  a  small  part  of  that  taking  place 
in  a  horizontal  direction  ;  it  increases  at  times  of  convectional  movements. 

120.  Hill  and  mountain  observatories.  The  observations  described  above 
suffice  to  determine  the  characteristic  movement  of  the  wind  about  the  place 
of  observation  ;  but  in  studying  the  general  movements  of  the  atmosphere,  it 
is  desirable  to  exclude  the  local  shifts  of  the  surface  winds,  and  for  this 
purpose,  observations  on  hills  or  lofty  towers  are  very  useful.  The  observatories 
established  by  Mr.  A.  L.  Rotch,  on  Blue  Hill,  near  Boston,  635  feet  above  the 
sea,  and  by  Mr.  W.  L.  Childs  on  Mt.  <Wantastiquet,  K.  H.,  opposite  Brattle- 
boro,  Vt.,  at  a  height  of  1076  feet  above  the  adjacent  valley  and  1364  feet 
above  sea-level,  are  of  much  interest  in  this  respect.  The  Ejffel  tower, 
990  feet  high,  erected  in  Paris  for  the  Exposition  of  1889,  gave  a  series  of 
particularly  instructive  meteorological  records,  inasmuch  as  its  slender  form 
produced  practically  no  disturbance  in  the  conditions  of  the  air  around  it ; 
while  there  is  reason  to  think  that  hills  and  mountains,  projecting  into  the 
atmosphere  in  large  mass,  cause  a  somewhat  more  hurried  flow  of  the  wind 
over  their  summits  than  is  found  from  the  velocities  of  floating  clouds  at  the 
same  height  in  the  free  air.  Yet  it  is  to  mountain  observatories  at  great 
heights  that  we  owe  at  present  the  most  definite  information  concerning  the 
movements  and  other  physical  features  of  the  upper  strata  of  the  atmosphere. 
Records  from  balloons  are  temporary ;  records  from  clouds  refer  only  to 
direction  and  velocity  of  movement,  and  are  moreover  not  obtainable  in  clear 
or  in  heavily  clouded  weather.  Records  on  mountains  may  be  maintained 
continuously  and  completely,  although  involving  great  expense.  The  mountain 
observatories  established  by  our  Signal  Service,  the  official  predecessor  of  our 
present  Weather  Bureau,  on  Mt.  Washington,  N.  H.,  in  1871,  at  an  altitude  of 
ni'7')  teet.  and  on  Pikes  Peak.  ('(.I....  in  1873,  at  an  altitude  of  14,134  feet,  have 
yielded  results  of  great  scientific  value  ;  but  on  account  of  their  heavy  expense 
and  their  relatively  small  value  in  the  daily  work  of  the  Service  in  predicting 
the  weather,  these  were  discontinued  in  1887  and  1888.  The  Lick  Observatory, 
on  Mt.  Hamilton,  Cal.,  4400  feet  above  sea-level,  maintains  a  full  meteorological 
record.  Automatic  records  are  maintained  by  the  Harvard  College  Observatory, 
on  Mt.  Chachani,  Peru,  at  an  altitude  of  1  (>,r>50  feet. 

The  most  important  of  the  mountain  observatories  in  Europe  are  named  in 
the  following  table. 


THE   PRESSURE    AND    CIRCULATION    OF    THE    ATMOSPHERE. 


97 


PEAK. 

MOUNTAIN  RANGE. 

COUNTRY. 

HEIGHT  ix  FEET. 

Jirii  Nevis           .     .     •     • 

Scotland  

4407 

Brocken                              « 

Hartz 

N.  Germany 

3743 

Schneekoppe      .... 
Wendelstein 

Riesen  Gebirge     .     . 
Alps            .        * 

Germany     .... 
Bavaria 

6246 
5669 

HochObir     
Sonnblick                     .     . 

E.  Alps  
E.  Alps  

Austria        .... 
Austria  .... 

7047 
10  155 

Sentis                                 • 

Alps  . 

Switzerland 

8215 

France    

4800 

Pic  du  Midi                  .     . 

Pyrenees     .... 

France    

9381 

121.  Wind  observations.  It  is  customary  to  make  observations  of  the 
wind  at  the  hours  selected  for  other  observations,  as  of  temperature  or  press- 
ure. It  should  be  noted  that  the  surroundings  of  an  observer  have  a  strong 
influence  on  the  accuracy  of  the  wind-record.  In  a  city,  the  wind  is  continually 
thrown  into  irregular  gusts  and  whirls  by  the  many  uneven  obstructions  that 
it  must  pass  over ;  in  a  valley,  the  velocity  is  reduced  and  the  direction  is 
altered  by  the  protecting  hillsides.  On  an  open  prairie,  there  is  good  oppor- 
tunity of  securing  comparable  results ;  but  even  then,  unless  the  vanes  and 
anemometers  of  different  observers  are  placed  at  the  same  height  above  the 
ground,  the  results  are  not  closely  accordant ;  for  the  wind  is  always  much 
retarded  by  the  resistances  felt  near  the  ground,  and  its  velocity  decreases 
rapidly  as  one  approaches  the  surface  of  the  land.  At  sea,  the  velocity  of  the 
wind  is  much  greater  than  on  the  continents,  and  it  is  probable  that  the 
increase  in  the  velocity  with  height  is  much  slower  than  on  land. 

No  standard  height  for  vanes  and  anemometers  has  yet  been  adopted, 
because  it  is  generally  impossible  to  conform  to  any  prescribed  rule  in  this 
respect ;  but  a  height  of  at  least  70  feet  above  the  open  ground  is  strongly 
recommended. 

The  increase  of  the  velocity  of  the  wind  at  considerable  elevations  has  been 
determined  by  observations  of  clouds  (Section  212) ;  the  results  of  such 
measurements  at  Blue  Hill  Observatory,  near  Boston,  Mass.,  may  be  intro- 
duced in  this  connection. 


MEAN    CLOUD    VELOCITIES    AT    VARIOUS    ALTITUDES  : 

OBSERVATORY. 


BLUE    HILL 


200 

1000 

3000 

5000 

7000 

9000 

11,000 

Altitude,  meters  .          ... 

to 

to 

to 

to 

to 

to 

to 

1000 

3000 

5000 

7000 

9000 

11,000 

13,000 

Mean  velocity  in  (  Summer  . 

7.5 

8.2 

10.6 

19.1 

23.5 

31.1 

35.2 

met.  per  sec.       1  Winter     . 

8.8 

14.7 

21.6 

49.3 

54.0 

98 


ELEMENTARY    METEOROLOGY. 


122.  Reduction  of  observations.  Wind  observations  are  commonly  reduced 
by  counting  the  number  of  times  the  various  directions  are  recorded,  and 
averaging  these  and  the  corresponding  velocities  with  respect  to  the  hour,  the 
month  and  the  year.  The  percentage  of  calms  in  the  total  number  of  observa- 
tions should  be  determined.  More  elaborate  reductions  require  an  analysis  of 
directions  and  velocities,  so  that  the  resultant  movement  of  the  air  may  be 
determined;  but  this  is  not  a  useful  method  where  the  direction  changes 
frequently  and  irregularly,  as  with  us. 

In  illustration  of  hourly  observations  of  the  wind,  reference  may  be  made 
to  Fig.  44,  showing  the  average  direction  of  the  wind  for  every  hour  of  the 
day  during  the  month  of  July,  1882,  at  the  Lake  Crib,  or  tower  from  which 
the  water  supply  was  taken  for  the  city  of  Chicago  from  Lake  Michigan ;  the 
regular  veering  of  the  lake  breeze  by  day  into  the  land  breeze  by  night  is  thus 
exhibited  to  a  nicety. 

The  average  annual  frequency  of  the  winds  at  Kinderhook  in  the  Hudson 
valley,  trending  north  and  south,  and  at  Utica  in  the  Mohawk  valley  trending 
east  and  west,  both  in  the  State  of  New  York  and  a  little  over  a  hundred  miles 

apart,  are  graphically  pre- 
sented in  Figs.  29  and  30 ; 
this  illustrates  very  clearly 
the  control  exerted  by  even 
these  broad  valleys  on  the 
course  of  the  prevailing 
winds. 

In  the  monthly  reports 
from  an  observing  station, 
it  is  customary  to  give  the 
prevalent  direction  or  direc- 
tions of  the  wind,  and  the 
total  wind  movement  for 

that  time ;  thus  for  January,  1885,  Pikes  Peak,  Colo.,  altitude  14,134  feet, 
had  winds  prevailingly  from  the  northwest,  with  a  total  movement  of  13,816 
miles  ;  Sandy  Hook,  at  the  entrance  to  New  York  harbor,  with  an  anemometer 
close  to  sea-level,  also  had  winds  generally  from  the  northwest,  with  a  total 
movement  of  14,932  miles.  The  exam  pi  ••  is  taken  from  the  Monthly  Weather 
Review  of  the  Weather  Service,  and  is  interesting  in  showing  an  exceptional 
greater  activity  of  the  surface  winds  than  of  tin-  upper  currents  for  the  month 
in  question  ;  but  if  the  resultant  of  all  the  movements  of  the  wind  were  taken, 
h  is  probable  that  the  back  and  forward  winds  of  Sandy  Hook  would  exhibit 
a  smaller  general  progression  of  the  air  in  any  single  direction,  while  on  l'ik<  s 
Peak  nearly  all  the  movement  was  in  OIK-  direction,  showing  a  great  forward 
movement  of  the  atmosphere  over  the  mountain  summit. 


FIG.  29. 


FIG.  30. 


THE   PRESSURE    AND    CIRCULATION    OF    THE    ATMOSPHERE.  99 

Ocean  charts  for  the  use  of  mariners  have  been  prepared  by  the  Hyd.ro- 
graphic  Offices  of  different  countries,  showing  among  other  data  the  relative 
frequency  and  average  force  of  the  winds  from  the  different  directions  during 
the  several  months  and  for  the  separate  latitude  and  longitude  "  squares  "  of 
the  ocean  areas.  The  parts  of  the  oceans  frequented  by  numerous  vessels  are 
well  explored  in  this  respect ;  but  there  are  still  large  areas  where  the  knowl- 
edge of  maritime  meteorology  is  deficient. 

123.  The  general  winds  of  the  world  are  best  studied  on  the  seasonal 
charts  of  pressure  (Charts  V  and  VI).  As  the  winds  of  the  southern  or 
austral  hemisphere  are  more  easily  generalized  than  those  of  the  northern  or 
boreal,  they  will  be  most  frequently  referred  to  in  this  general  account.  A 
fuller  account  of  the  winds  of  different  regions  will  be  given  in  the  next 
chapter. 

In  July  the  winds  between  the  southern  tropical  belt  of  high  pressure  and 
the  equatorial  belt  of  low  pressure  blow  steadily  from  the  southeast ;  these 
are  the  southern  members  of  the  trade  winds.  In  the  great  circumpolar  area 
beyond  the  high-pressure  belt,  the  winds  blow  briskly  from  the  northwest  or 
west-northwest,  but  are  much  confused  by  stormy  interruptions  ;  these  will  be 
railed  the  prevailing  ivesterly  winds.  In  the  middle  latitudes  of  the  south 
temperate  zone  the  winds  are  so  boisterous  as  to  have  gained  the  name  of  the 
"roaring  forties."  The  directions  here  named  are  much  affected  on  and  in 
the  neighborhood  of  the  land ;  but  as  the  austral  continents  are  relatively 
small,  the  winds  of  that  half  of  the  world  blow  for  the  most  part  as  has 
been  briefly  stated.  In  January  the  division  between  the  westerlies  and  the 
trades  is  less  distinct  than  in  July ;  in  the  South  Atlantic,  for  example,  when 
the  continuity  of  the  tropical  high-pressure  belt  is  broken,  the  winds  seem  to 
circulate  around  a  central  area  of  high  pressure;  while  in  July,  when  the 
tropical  high-pressure  belt  stretches  across  land  and  water,  it  separates  the 
two  members  of  the  wind  system  by  a  more  linear  division.  In  July,  more- 
over, the  austral  winds  on  the  whole  blow  faster  than  in  January. 

In  the  northern  hemisphere  the  winds  are  in  a  general  way  symmetrical 
with  those  of  the  southern.  There  is  a  northeast  trade  wind  over  the  zone 
from  the  tropical  high-pressure  belt  to  the  belt  of  low  pressure  around  the 
equator  ;  and  on  the  oceans  at  least  there  is  a  prevalent  west-southwest  wind 
over  a  considerable  part  of  the  area  from  the  tropical  belt  towards  the  pole, 
with  higher  velocities  in  January  than  in  July  ;  in  latitudes  above  60°  N. 
northeast  winds  are  frequently  recorded.  But  as  the  lands  here  occupy  so 
much  greater  a  share  of  the  total  area  than  in  the  southern  hemisphere,  the 
irregular  winds  that  they  produce  greatly  distort  the  boreal  wind  system. 
Tin*  simplest  statement  of  this  distortion  is  that  the  winds  tend  to  blow 
obliquely  outward  from  the  continents  in  January,  and  obliquely  inward  in 


100  ELEMENTARY  METEOROLOGY. 

July ;  but  this  tendency  is  greatly  modified  by  the  presence  of  the  prevailing 
westerly  winds  in  the  latitudes  where  the  continents  are  broadest.  A  similar 
effect  is  shown  in  the  seasonal  variation  of  the  winds  over  Australia. 

The  axes  of  the  tropical  high-pressure  belts  and  of  the  equatorial  low- 
pressure  belt,  where  the  gradients  are  zero,  are  characterized,  especially  on  the 
oceans,  by  weak  and  variable  winds  with  not  infrequent  calms,  in  strong 
contrast  to  the  continuous  movement  of  the  steady  trades  on  the  one  hand,  or 
to  the  stormy  westerly  winds  on  the  other.  The  equatorial  belt  in  particular 
is  marked  by  frequent  calms,  called  the  doldrums,  which  migrate  north  and 
south  with  the  barometric  equator,  which  in  its  turn  follows  the  heat  equator ; 
but  it  is  an  exaggeration  to  describe  the  doldrums  as  a  belt  of  continuous 
calms.  The  light  winds  and  calms  of  the  tropical  belts  mark  the  "  horse 
latitudes,"  and  these  also  have  a  slight  annual  migration  north  and  south 
with  the  sun. 

COMPARISON  OF  THE  CONSEQUENCES  OF  THE  CONVECTIONAL  THEORY 
WITH  THE  FACTS  OF  PRESSURE  AND  WINDS. 

124.  General  relation  of  winds  and  pressures.     The  winds  show  a  distinct 
tendency  to  blow  from  areas  of  high  pressure  towards  areas  of  low  pressure  ; 
they  blow  faster  on  the  steeper  gradients  of  winter  than  on  the  fainter  ones 
of  summer ;  they  are  boisterous  on  the  steep  gradients  of  the  south  temperate 
zone,  and  they  weaken  almost  to  stagnation  where  the  gradients  disappear 
along  the  axes  of  the  pressure  belts.     According  to  the  general  principles  of 
convectional  circulation,  as  stated  in  Section  93,  it  was  expected  that  the 
winds  should  follow  the  line  of  the  gradient ;  but  this  is  clearly  not  the  fact. 
The  boreal  winds  turn  to  the  right ;  the  austral  winds  turn  to  the  left  of  the 
gradient. 

125.  Agreements  and  disagreements.      This  general  review  of  the  distri- 
bution of  pressures  and  circulation  of  the  winds  has  discovered  two  particulars 
in  which  the  expected  arrangement  of  pressures  and  motions  are  contradicted 
by  the  facts.     The  polar  pressures  are  not  high,  but  low  ;  and  the  pressure  is 
highest  around  the  tropics,  where  intermediate  values  were  expected  :    the 
winds  do  not  flow  along  the  gradients,  but  turn  systematically  to  one  side  or 
the  other.     Otherwise,  the  consequences  of  the  convectional  theory  accord 
with  the  facts. 

When  an  investigation  reaches  this  stage,  the  student  may  review  its 
progress  in  some  such  way  as  this  :  Either  the  suggested  explanation  by 
means  of  convection  is  fundamentally  wrong,  in  which  case  it  should  l>e 
replaced  by  another ;  or  the  explanation  needs  some  supplements  by  which  to 
account  for  the  polar  low  pressures  and  the  oblique  course  of  the  winds.  It 


THE    PKESSTJKE   AND    CIRCULATION    OF   THE   ATMOSPHERE. 

ran  hardly  be  supposed  that  an  explanation  as  well  grounded  on  accepted 
physical  laws  as  the  one  outlined  in  the  statement  of  the  general  principles  of 
convectional  motion  should  be  entirely  wrong;  moreover,  such  facts  as  the 
seasonal  variations  of  pressure  and  winds  over  the  continents  and  the 
frequency  of  calms  along  the  axes  of  the  pressure  belts  on  the  oceans  are 
much  in  its  favor.  It  should  not  be  discarded  until  the  possibility  of  supple- 
menting its  deficiencies  has  been  very  carefully  tested. 

Next  it  might  be  asked  whether  each  one  of  the  two  classes  of  deficiencies 
requires  special  supplementary  explanation  ;  or  whether  one  class  may  not  be 
related  to  the  other  as  a  cause  is  to  an  effect ;  for  in  that  case  a  single  supple- 
ment that  would  explain  the  first  would  explain  the  second  also.  Let  us 
consider  if  the  oblique  course  of  the  winds  can  account  for  the  unexplained 
distribution  of  pressure  at  the  poles  and  the  tropics. 

126.  Low  polar  pressure  caused  by  the  prevailing    westerly  winds. 
Brief  mention  was  made  in  Section  13  of  the  flattening  of  the  earth  at  the 
poles  by  the  centrifugal  force  of   its  diurnal  rotation,  once  in  twenty-four 
hours.     If  it  should  rotate  faster,  it  would  be  more  flattened;  if  slower,  it 
would  be  more  nearly  spherical.     Look  now  at  the  winds  of  the  southern 
hemisphere,  from  the  tropical  belt  of  high  pressure  to  the  pole.     They  are 
moving  eastward  over  the  earth's  surface  in  a  great  whirl  around  the  south 
pole.     The  upper  winds,  carrying  the  clouds,  run  even  faster  eastward  than 
the  surface  winds.     As  a  whole,  they  accomplish  a  revolution   around  the 
earth's  axis  in  less  than  twenty-four  hours  ;  and  hence  their  centrifugal  force 
must  be  greater  than  that  of  the  earth.     May  it  not  be  that  the  expected  high 
pressure  at  the  pole  is  reduced  to  low  pressure  by  the  excessive  centrifugal 
force  of  the  circumpolar  whirl,  and  that  the  air  thus  withheld  from  the  polar 
region  is  found  in  the  tropical  belt  of   high  pressure  ?     This  suggestion  is 
certainly  too  important  to  be  neglected ;  it  is  plausible  enough  to  warrant  its 
provisional  acceptance,  while  search  is  made  for  the  cause  of  the  deflection  of 
the  winds  from  the  gradients,  whereby  the  circumpolar  whirl  is  produced. 

THE  EFFECTS  OF  THE  EARTH'S  ROTATION. 

127.  The  deflecting  force  of  the  earth's  rotation.     The  cause  of  the 
deflection  of   the  winds  from  the  gradients  is  to  be  found  in  the  earth's 
rotation.     It  may  be  easily  explained  and  illustrated  by  experiment  (see  Sect. 
133)  that  the  winds  cannot  follow  the  gradients,  because  there  arises  from  the 
earth's  rotation  a  force 1  that  tends  to  deflect  all  horizontal  motions,  of  what- 

1  Although  always  spoken  of  as  a  "force,"  this  term  implies  a  misconception  of  the  same 
kind  as  that  which  often  embarrasses  the  understanding  of  "centrifugal  force."  A  body 
movine  without  friction  over  the  surface  of  the  earth  tends  to  move  in  the  direction  of  its 


E  L  K  M  K  N  T ARY    METEOROLOGY. 


direction,  to  the  right  in  the. northern  hemisphere,  and  to  the  left  in  the 
southern :  the  deflecting  force  is  proportionate  to  the  velocity  of  motion,  and 
increases  from  zero  at  the  equator  to  a  maximum  value  at  either  pole. 

The  value  of  the  deflecting  force,  in  terms  of  the  weight  of  the  moving 
body,  may  be  determined  for  any  latitude  by  multiplying  the  appropriate 
factor  in  the  following  table  by  the  velocity  of  motion,  expressed  in  miles  per 
hour. 


LATITUDE. 

FACTOR. 

LATITUDE. 

FACTOR. 

0°  
10° 

0.00,000,000 
115 

50°     
60°          

0.00,000,;  •>(>'.' 
570 

20° 

228 

70°                    . 

02-") 

30°  
40° 

:;;;:; 

427 

80°    
00°          .               .... 

655 

665 

128.  Hadley's  theory  of  the  effect  of  the  earth's  rotation :  1735.  The 
introduction  of  this  important  principle  into  our  science  lias  been  slow,  and 
even  to  this  day  it  is  not  properly  appreciated  by  many  students  of  the  subject. 
The  oblique  movement  of  the  trade-winds  was  known,  from  the  accounts  of 
navigators,  to  Halley,  a  famous  English  astronomer,  who  tried  to  explain  it  in 
1686  as  a  result  of  the  (apparent)  westward  movement  of  the  sun  around  the 
earth.  In  1735  this  was  shown  to  be  wrong  by  Hadley,  another  English 
astronomer,  who  introduced  the  first  reference  to  the  real  cause ;  but  his  essay 
was  generally  overlooked  for  the  greater  part  of  the  last  century,  until  the 
same  idea  had  occurred  to  several  other  investigators.  The  explanation  still 
generally  current  follows  that  given  by  Hadley,  in  brief  as  follows  :  If  a  mass 
of  air  moves  from  latitude  30°  north  towards  the  rarefied  belt  of  heated  air 
around  the  equator,  it  advances  upon  latitudes  whose  eastward  rotary  velocity 
is  greater  and  greater,  and  in  consequence  of  this,  the  air  lags  behind,  ami 
hence  appears  'as  an  oblique  northeast  wind  ;  indeed,  if  it  were  not  for  the 
1'rirtion  with  tin-  surface  of  the  land  and  sea,  by  which  the  advancing  air 
continually  acquires  something  of  the  eastward  motion  of  the  latitudes  that  it 
enters,  there  should  be  a  violent  westward  hurricane,  of  a  hundred  or  more 
mill's  an  hour  at  the  equator,  according  to  this  theory.  Hadley  did  not  apply 

first  impulse.  We  live  on  the  earth's  surface,  unconscious  of  its  rotary  movement,  and  < •<»!- 
frequently  persuaded  that  any  straight  line  hold*  a  fixed  direction.  Hence,  when  a  I'm - 
moving  body  (such  as  a  frec-swin^inir  pendulum,  as  in  Foiicaulfs  experiment)  turns  aside 
from  its  first  line  of  movement,  we  assume  that  its  dim-lion  has  l.een  changed  l.y  some 
deflecting  force.  In  reality,  the  free-moving  body  perseveres  in  its  original  direction,  in 
virtue  of  \\-  ii,,  rti.i  :  it  is  the  apparently  fixed  line  of  reference  iliat  is  ,  hairjii-  its  direction, 
in  virtue  of  the  earth's  rotation.  The  --detlectii,  is  therefore  ,,idy  tin-  inertia- 

i nee  that  a  free-moving  body  exerts  :iLrain-i  a  constraining  force  that   ur^es  it,  to  move 
in  what  we  call  a  straight  line  or  a  fixed  direction. 


THE    PRESSURE   AND   CIRCULATION    OF    THE   ATMOSPHERE.          103 

his  explanation  to  the  prevailing  westerly  winds ;  but  it  has  been  applied  to 
them  by  his  followers,  who  teach  that  as  these  winds  advance  northward  from 
tne  tropical  belt,  they  enter  latitudes  whose  eastward  velocity  is  less  and  less  ; 
and  in  consequence  of  this  the  winds  run  ahead  of  the  surface  and  gain  a 
direction  from  the  southwest  or  west-southwest. 

This  explanation  contains  two  serious  errors,  which  are  here  referred  to 
because  they  have  gained  general  currency.  Hadley's  explanation  implies  that 
there  is  no  effect  produced  on  motions  to  the  east  or  west,  while  as  stated 
above,  the  deflective  force  arising  from  the  earth's  rotation  is  independent  of 
the  direction  of  motion.  Again,  Hadley's  explanation  teaches  that  a  body 
moving  towards  the  equator  continually  lags  westward,  so  that  if  friction  had 
no  effect  it  would  attain  a  great  velocity  to  the  west  when  it  reached  the 
equator.  This  is  wrong ;  the  lagging,  if  such  an  expression  is  introduced  at 
all,  cannot  be  continually  in  one  direction,,  as  to  the  west,  but  can  only  be  at 
right  angles  to  the  momentary  direction  of  motion,  and  hence  can  produce  no 
effect  on  the  velocity.  If  a  body  were  given  a  velocity  of  25  miles  an  hour  to 
the  south  when  in  latitude  30°  N.,  and  was  supposed  to  move  without 
friction  over  a  level  surface,  it  would  continue  to  move  at  the  same  moderate 
rate  whatever  latitude  it  reached ;  while  Hadley's  explanation  would  give 
it  a.  velocity  of  a  hundred  or  more  miles  westward  at  the  equator.  A  proper 
understanding  of  the  true  value  and  action  of  the  deflective  force  should 
therefore  be  introduced  into  the  popular  teaching  of  meteorology. 

The  deflective  effect  of  the  earth's  rotation  was  worked  out  by  various 
mathematicians  in  the  early  part  of  this  century ;  but  its  first  proper  applica- 
tion to  meteorology  was  in  1843,  when  Charles  Tracy,  then  a  young  graduate 
of  Yale  College,  afterwards  a  well-known  lawyer  in  New  York  City,  published 
a  brief  article  on  the  subject,  showing  how  the  rotation  of  storms,  which  was 
at  that  time  attracting  much  attention,  should  necessarily  result  from  the 
rotation  of  the  earth.  This  article  was  curiously  overlooked ;  no  reierence 
was  made  to  it  till  nearly  forty  years  after  its  publication,  and  in  the  mean- 
time the  question  had  been  much  more  fully  investigated  by  others. 

129.  FerrePs  theory  of  the  effects  of  the  earth's  rotation :  1856.  Ferrel's 
studies  of  the  effects  of  the  earth's  rotation  on  the  circulation  of  the  atmo- 
sphere were  begun  in  1856 ;  they  were  more  fully  expanded  in  later  years, 
and  it  is  not  too  much  to  say  that  they  have  worked  a  revolution  in  the  science 
of  meteorology,  Ferrel  was  at  that  time  teaching  school  in  Nashville,  Tenn. ; 
he  was  a  self-taught  mathematician  of  remarkable  originality.  Lpon  reading 
a  statement  of  the  erroneous  theories  of  the  winds  then  in  vogue,  he  studied 
the  matter  out  for  himself  and  produced  the  first  rational  theory  of  the  general 
circulation  of  the  atmosphere.  At  that  time  the  prevailing  low  pressure  near 
the  poles  was  coming  into  notice,  particularly  from  the  observations  made  in 


104 


ELEMENTARY    METEOROLOGY. 


the  Antarctic  voyages  of  Ross  and  Wilkes ;  but  it  received  no  proper  explana- 
tion until  solved  by  Ferrel,  who  showed  conclusively  that  it  must  result  from 
the  establishment  of  an  interchanging  convectional  circulation  between  the 
equator  and  the  poles  on  a  rotating  earth. 

It  is  impossible  to  present  in  brief  outline  and  in  non-mathematical  form 
an  adequate  statement  of  Ferrel's  theory  ;  but  the  following  paragraphs  may 
serve  to  place  its  essential  features  before  the  student. 

130.  Motion  on  the  rotating  earth  without  friction.  If  a  body  be 
supposed  to  move  without  friction  on  the  level  surface  of  a  rotating  globe,  a 
single  impulse  would  give  it  a  perpetual  motion ;  but  the  motion  could  not  be 
along  a  straight  path.  It  would  continually  be  deflected  to  one  side  of  its 
momentary  path,  to  the  right  in  our  hemisphere,  to  the  left  in  the  other,  and 
with  a  force  dependent  on  its  velocity  and  on  its  latitude ;  but  independent 
of  its  direction  of  motion.  Its  path  would  always  be  curved  in  a  systematic 
manner ;  the  curvature  would  be  sharper  for  slow  motions  than  for  rapid 
motions,  and  sharper  in  high  latitudes  than  near  the  equator.  The  following 
table  will  give  some  idea  of  the  rate  at  which  a  moving  body  tends  to  turn 
from  a  straight  line  on  a  sphere  rotating  once  in  twenty-four  hours  ;  the  first 
column  being  its  velocity,  the  other  columns  giving  the  radius  of  curvature  of 
the  path  in  which  it  would  move  at  several  different  latitudes. 

RADIUS  OF  CURVATURE  (IN  MILES)  FOR  FRICTIONLESS  MOTION  ON  THE 

EARTH'S  SURFACE. 


Latitude 

O9 

6° 

ID 

20° 

30° 

40° 

60° 

60° 

70° 

80° 

90° 

20  miles  an  hour  

00 

880 

442 

224 

153 

119 

100 

88 

82 

78 

77 

10  miles  an  hour  

CO 

440 

221 

112 

76 

59 

50 

44 

41 

39 

38 

5  miles  an  hour  

00 

220 

110 

66 

38 

30 

25 

22 

20 

19 

19 

A  body  once  set  in  motion  under  these  conditions  would  continue  moving 
forever,  always  changing  its  direction  but  never  changing  its  velocity.  If  it 
were  given  a  velocity  of  20  miles  an  hour  in  any  direction  at  latitude  30°,  it 
would  describe  a  series  of  overlapping  loops,  gradually  carrying  it  westward 
around  the  earth,  but  never  passing  outside  of  the  parallels  of  20°  or  40°.  If 
it  were  given  a  velocity  of  five  or  more  miles  an  hour  eastward  at  latitude  5*, 
it  would  describe  a  scalloped  path,  oscillating  back  and  forth  across  the 
equator,  but  never  escaping  beyond  latitude  5°  in  either  hemisphere. 

131.  Movement  of  the  air  on  gravitative  gradients.  The  imaginary  case 
of  the  preceding  paragraph  does  not  apply  directly  to  the  case  of  the  winds, 
for  they  are  not  acted  on  by  a  single  initial  impulse,  but  by  a  continual 
gravitative  acceleration,  according  to  their  gradient ;  and  they  are  more  or  less 


THE    I'UESSrRE    AND    CIRCULATION    OF    THE    ATMOSPHERE. 


105 


FIG.  31. 


affected  by  friction  and  other  resistances.  The  practical  question  then 
is  :  What  will  be  the  course  of  the  general  winds  on  a  given  gradient  at 
a  given  latitude  ;  it  being  understood  that  the  conditions  of  steady  motion 
have  long  ago  been  reached,  as  far  as  the  average  value  of  the  gradients  is 
concerned. 

Consider  for  example  the  northeast  trade  wind  of  our  hemisphere,  A  W, 
Fig.  31.  It  is  urged  by  a  small  component  of  gravity,  AC,  to  move  towards 
the  equator  ;  but  by  reason  of  its  velocity  of  about  twenty 
miles  an  hour  in  latitude  20°  N.  it  must  experience  a 
certain  right-handed  deflective  force,  represented  by  the 
arrow  AD,  at  right  angles  to  A  W.  The  force,  A  C,  expends 
one  of  its  components,  AF,  in  overcoming  the  deflective 
force  (inertia),  AD.  The  other  component,  AB,  must  just 
equal  and  oppose  the  sum  of  all  the  resistances,  AE,  that 
are  excited  by  the  velocity,  A  W;  and  in  such  a  condition 
of  equilibrium  all  the  permanent  winds  of  the  world  must 
blow.  They  always  adjust  their  velocity  and  direction  so 
us  to  bring-  about  an  equilibrium  among  the  acting  forces. 

The  prevailing  westerlies  of  the  far  southern  latitudes  may  receive  a 
similar  explanation  ;  their  austral  deflection  turning  them  to  the  left  of  their 
gradients. 

Consider  next  the  condition  of  the  equatorial  overflow  that  is  inferred  to 
run  from  the  warm,  expanded  air  above  the  equator  towards  the  cold,  com- 
pressed air  of  the  south  polar  region.  What  course  must  it  have  in  latitude 
25°  or  30°  S.  and  at  a  great  height  above  sea-level  ?  An  examination  of  the 
gradients  of  this  part  of  the  atmosphere,  as  illustrated  in  Fig.  26,  shows  that 

the  accelerating  force  there  must  be 
much  greater  than  that  by  which  the 
trade  winds  are  impelled.  It  may  be 
represented  by  AC,  Fig.  32.  More- 
over the  resistance,  AE,  which  the 
currents  of  the  upper  air  encounter 
must  be  comparatively  small.  Now 
we  must  suppose  that  as  the  overflow 
current  has  certainly  ages  ago  acquired 
the  condition  of  steady  motion,  this 
small  resistance  is  the  equal  and 
opposite  of  an  equally  small  residual  force,  AB,  left  over  from  the  gravi- 
tative  acceleration,  AC,  after  its  greater  part  has  been  expended  in  overcom- 
ing the  deflective  force  (inertia),  AD.  The  part  of  gravitative  force  that  acts 
effectively  on  a  baric  gradient  is  therefore  even  less  than  was  indicated  in 
Section  113.  We  may  next  proceed  to  determine  the  deflective  force  ;  and  then 


F+C 


FIG.  32. 


106  ELEMENTARY    METEOROLOGY. 

knowing  the  deflective  force,  the  velocity  and  direction  of  the  motion  that 
arouses  it  may  be  found. 

The  deflective  force  may  be  found  by  completing  the  parallelogram, 
of  which  A  C  is  a  side  and  AB  is  a  diagonal ;  the  angle,  DA B,  being  90°. 
The  deflective  force,  AD,  is  therefore  almost  equal  and  opposite  to  the  grav- 
itative  force,  AC.  The  overflow  must  then  be  represented  by  AW,  showing 
it  to  have  a  high  velocity  and  a  direction  but  little  south  of  east.  In  no  other 
case  can  the  deflective  force  have  its  appropriate  direction  and  value  with 
respect  to  the  current  that  produces  it.  The  greater  part  of  the  high  level 
overflow  from  the  equator  must  therefore  run  at  a  high  velocity  in  a  direction 
almost  from  west  to  east,  but  a  little  inclined  toward  the  pole  ;  in  no  other 
direction  can  the  conditions  of  steady  motion  be  reached  in  the  presence  of  the 
small  resistances  of  the  upper  air.  All  that  is  known  of  the  high-level  currents 
in  middle  latitudes  from  observations  of  clouds  confirms  this  explanation  in  a 
striking  manner. 

132.  Deflection  of  the  winds  from  the  gradients.     One  of  the  deficiencies 
in  the  convectional  theory  of  the  winds,  pointed  out  in  Section  125,  is  thus 
satisfactorily  accounted  for.      Not  only  the  trade  winds  and  the  prevailing 
westerlies  follow  the  explanation  of  the  preceding  section,  but  all  the  smaller 
members  of  the  general  circulation  also  turn  aside  from  their  gradients,  as 
seen  in  the  spiral  outflow  from  the   South  Atlantic  area  of  tropical  high 
pressure  in  January  ;  or  from  that  of  the  North  Atlantic  in  July  ;  or  from  the 
Asiatic  area  of  continental  high  pressure  in  January  ;  or  as  seen  again  in  the 
spiral  inflow  towards  the  area  of  low  pressure  over  the  North  Atlantic  near 
Iceland  in  January  ;  or  over  Asia  in  July  ;  or  over   Australia  in  January. 
All  these  and    many   other  examples  to  be  met  on   subsequent   pages   are 
reconciled    when   the  theory  not  only  takes    account  of   the    interaction  of 
insolation  and  gravity,  but  when  it  includes  the  effect  of  the  earth's  rotation 
as  well. 

Before  advancing  further  in  the  discussion  of  the  circumpolar  whirl,  an 
experimental  review  may  be  made  of  the  points  thus  far  learned. 

133,  Experimental  illustration  of  the  deflective  effect  of  the  earth's 
rotation.     Stretch  a  smooth  sheet  of  paper  over  a  circular  table,  two  or  three 
feet  in  diameter,  supported  at  the  center  on  a  vertical  axis  on  which  it  may 
rotate  in  either  direction.     Lay  a  marble,  dipped  in  ink,  at  the  center  of  the 
stationary  table.     It  remains  at  rest.     This  corresponds  to  the  conditions  of 
level  isobaric  surfaces,  on  which  there  could  l>e  no  winds. 

•  the  marble  rolling  by  a  light  blow  ;  it  will  trace  its  path  by  dots  of  ink 
alon^  ;i  straight  radial  line;  this  corresponds  roughly  to  the  case  of  motion 
started  by  an  initial  impulse,  on  a  smooth  non-rotating  earth.  A  body 


THE    PRESSURE   AND    CIRCULATION    OF    THE    ATMOSPHERE.  107 

under  such  conditions  on  the  earth's  surface  would  follow  a  straight  path, 
always  moving  in  the  same  great  circle  on  which  it  started. 

Rotate  the  table  slowly  from  right  to  left,  and  set  the  marble  in  motion  as 
ho  lore  ;  it  will  describe  a  curved  path,  turning  to  the  right  of  the  radius  on 
which  its  motion  began.  This  corresponds  to  motion  started  in  the  northern 
hemisphere  by  an  initial  impulse  on  the  level  surface  of  a  rotating  earth. 
When  the  table  is  rotating  at  a  given  rate,  a  slow  motion  of  the  marble  causes 
a  sharp  curvature  in  its  path.  When  the  marble  is  moving  at  a  given  velocity, 
a  faster  rotation  of  the  table  (corresponding  to  a  higher  latitude  on  the  earth) 
causes  a  sharper  curvature  of  the  path.  These  results  correspond  essentially 
with  those  given  in  Section  130,  although  the  experiment  is  imperfect  from 
the  effects  of  friction. 

Tilt  the  table  slightly  on  a  hinge  at  the  axis,  so  that  it  shall  present  an 
inclined  surface,  although  it  may  still  rotate  on  a  vertical  axis.  Let  the  table 
stand  still,  and  release  the  marble  at  the  center.  It  will  describe  a  straight 
path  under  the  acceleration  of  the  component  of  gravity  which  acts  down  the 
inclination  of  the  surface.  This  corresponds  to  the  gravitative  acceleration  of 
the  wind  on  inclined  isobaric  surfaces  on  a  non-rotating  earth.  The  velocity 
attained  will  depend  on  the  inclination  of  the  table  and  on  the  resistances 
encountered.  At  a  given  inclination,  the  rougher  the  table,  the  slower  the 
velocity  when  steady  motion  is  gained.  This  corresponds  to  the  conditions 
of  steady  motion  in  a  simple  convectional  motion,  as  explained  in  Section  94. 
In  ordinary  experiments  the  table  is  too  smooth .  and  its  radius  too  small  for 
the  attainment  of  steady  motion. 

The  table  being  gently  tilted,  rotate  it  slowly  from  left  to  right ;  release 
the  marble  as  before,  and  it  will  describe  a  curved  path,  turning  to  the  left  of 
the  table  gradient.  This  correspond  to  the  actual  case  of  motion  of  the  air 
on  inclined  isobaric  surfaces  in  the  southern  hemisphere. 

After  the  marble  has  rolled  a  little  distance  from  the  center,  its  path 
maintains  an  almost  constant  deflection  from  the  gradient  of  the  table.  The 
larger  and  smoother  the  table,  the  better  this  may  be  seen.  This  corresponds 
to  the  conditions  of  steady  motion  under  the  action  of  gravitative  acceleration 
and  the  deflecting  force,  as  explained  in  Section  131.  If  the  rotation  of 
the  table  be  slow,  the  final  angle  of  deflection  will  be  small,  corresponding  to 
the  winds  in  low  latitudes,  where  they  do  not  turn  far  from  the  direction  of 
the  decrease  of  pressure.  If  the  rate  of  rotation  be  faster,  the  angle  of 
deflection  becomes  greater,  corresponding  to  the  strong  departures  of  the  wind 
from  the  gradient  in  high  latitudes.  If  the  table  be  smooth,  the  resistances 
to  motion  will  be  small,  and  the  angle  of  deflection  becomes  large,  as  in  the 
case  of  high-level  atmospheric  currents.  If  the  table  be  rougher,  a  consider- 
able value  is  required  in  the  forward-acting  resultant,  and  hence  the  deflection 
is  comparatively  small ;  this  being  the  case  of  the  lower  winds,  in  contact 


108  ELEMENTARY    MKTKoii()LO(iY. 

with  the  surface  of  the  ocean,  where  they  beat  against  the  waves ;  and  still 
more  where  they  blow  over  the  uneven  surface  of  the  land.  This  explains  the 
prevailing  difference  between  the  direction  of  the  surface  wind  and  that  of  the 
lower  clouds  at  a  height  of  one  or  two  thousand  feet ;  the  latter  as  a  rule  come, 
in  our  hemisphere,  from  a  point  somewhat  to  the  right  of  the  former,  show- 
ing that  their  deflection  from  the  gradient  is  somewhat  stronger  than  that  of 
the  surface  winds. 

It  is  not  at  first  apparent  why  the  rotating  table  employed  in  these  expert 
ments  may  be  compared  with  one  or  another  part  of  the  earth's  surface.  This 
may  be  made  clear  by  the  following  illustration. 

Several  circular  discs  of  paper,  an  inch  or  two  in  diameter  and  each 
marked  with  a  strong  diametral  line,  may  be  attached  to  a  terrestrial  globe  in 
different  latitudes.  Watch  the  diametral  line  on  one  of  the  discs  while  the 
globe  is  slowly  rotated ;  the  line  will  be  seen  to  change  its  direction ;  now 
pointing  to  one  part  of  the  room,  now  to  another.  In  other  words,  the  disc  is 
rotating  with  respect  to  its  center,  and  in  the  same  direction  as  the  globe 
rotates.  A  disc  near  the  pole  will  rotate  rapidly  ;  a  disc  near  the  equator  will 
turn  its  diameter  more  slowly  from  one  direction  to  another ;  a  disc  on  the 
equator  has  no  motion  of  rotation  with  respect  to  its  center ;  and  at  the 
equator  there  is  no  deflective  force.  These  discs  may  represent  the  table  of 
the  preceding  experiments.  The  marble  at  the  center  of  the  table  or  the  air 
at  any  point  on  the  earth  experiences  a  deflection  from  its  first  path  as  it 
moves  in  any  direction  from  the  starting  point ;  the  deflective  force  varies 
with  the  velocity  of  motion  and  with  the  rate  of  the  rotation  of  the  surface 
with  respect  to  its  center  ;  and  the  direction  taken  when  steady  motion  is 
attained  changes  accordingly. 

Hence,  whenever  a  mass  of  air  is  impelled  to  move,  as  by  the  introduction 
of  some  change  of  temperature  which  produces  a  gradient  in  the  previously 
level  isobaric  surfaces,  it  will  soon  turn  from  the  gradient  and  adjust  itself  to 
an  equilibrium  under  the  action  of  gravitative  acceleration  and  the  deflec- 
tive force.  In  motions  between  the  equator  and  the  poles,  where  the 
differences  of  temperature  were  long  since  introduced,  and  now  vary  only  by 
moderate  amounts  above  or  below  their  mean  value,  the  currents  of  the  atmo- 
sphere must  have  long  ago  attained  a  condition  of  steady  motion,  hurrying  a 
little  when  the  differences  of  temperature  increase  in  winter  and  steepen  the 
gradients  by  a  small  amount,  and  falling  to  lower  velocities  when  the  gradients 
are  weaker  in  summer.  Inasmuch  as  the  gradients  and  the  deflective  force 
vary  from  latitude  to  latitude,  it  follows  that  the  velocity  attained  by  the 
general  circulation  of  the  atmosphere  must  likewise  vary  between  the  equator 
and  poles  ;  but  at  every  latitude  a  temporary  equilibrium  is  taken,  to  be  lost 
only  as  the  wind  moves  into  a  new  position,  where  the  forces  acting  on  it 
change  their  relative  values. 


THE    PRESSURE    AND    CIRCULATION    OF    THE    ATMOSPHERE.          109 

134.  Analogy  with  an  eddy  in  water.  An  illustration  of  the  atmo- 
spheric whirl  mentioned  in  Sections  126  and  132  may  be  found  in  a  basin  of 
water  discharging  itself  by  a  vent  at  the  bottom.  If  the  water  stand  still 
when  the  vent  is  opened,  its  currents  will  move  in  radially  towards  the  center, 
and  there  descend  without  attaining  any  great  velocity  ;  but  if  a  gentle  rotary 
motion  be  given  to  the  water  before  the  vent  is  opened,  the  discharge  will 
require  a  much  longer  time,  and  will  be  deflected  so  as  to  form  a  rapidly 
whirling  central  eddy  or  vortex  of  increasing  velocity  towards  the  center, 
where  its  centrifugal  force  may  be  so  great  as  to  open  an  empty  core.  The 
analogy  of  this  case  with  that  of  the  circumpolar  whirl  of  our  atmosphere 
is  very  imperfect ;  but  it  serves  to  emphasize  the  great  value  and  strong  effect 
of  the  centrifugal  force  that  may  be  developed  in  such  a  vortex  ;  and  it  is  on 
such  a  centrifugal  force  that  we  are  counting  to  reduce  the  expected  high 
pressure  at  the  pole  into  the  actual  low  pressure. 

If  we  imagine  ourselves  looking  at  the  earth  so  that  the  south  pole 
appears  in  the  center,  while  the  equator  forms  the  marginal  circumference, 
then  the  great  equatorial  overflow,  rotating  with  the  earth  at  the  equator, 
may  be  likened  to  the  case  of  the  rotating  body  of  water  about  to  discharge 
itself  at  the  center  of  the  basin.  Let  us  examine  into  the  velocities  and 
accompanying  centrifugal  forces  that  would  be  gained  if  there  were  no  loss  by 
friction. 

Let  the  radius  of  the  basin  be  ten  inches  ;  let  the  linear  velocity  of 
rotation  at  the  margin  of  the  basin  be  one  foot  a  second.  As  the  water  is 
drawn  in  towards  the  center,  its  linear  velocity  of  rotation  will  increase  just 
as  much  as  its  radius  is  diminished  ?  this  is  in  accordance  with  a  well-known 
mechanical  principle,  called  the  conservation  of  areas  ;  because  the  area  swept 
over  in  a  given  time  by  a  radius  from  the  center  to  any  particle  of  water  is 
constant.  The  centrifugal  force  developed  by  a  rotating  body  varies  with  the 
square  of  the  velocity  of  rotation  divided  by  the  radius  of  rotaticn.  The 
following  table  exhibits  the  rapid  increase  of  centrifugal  force  as  the  center  of 
the  vortex  is  approached. 

CENTRIFUGAL  FORCE  IN  A  VORTEX. 


RADIUS. 

LINEAR  VELOCITY. 

CEXTRIF.  FORCB. 

10 
6 
1 

1 
2 
10 

A 

1 

100 

JL! 

100 

100,000 

It  is  manifest  that  even  a  less  excessive  centrifugal  force  close  to  the  axis 
of  the  eddy  may  suffice  to  open  an  empty  core,  whose  surface  is  everywhere  at 
right  angles  to  the  resultant  of  the  downward  gravitative  force  and  the  out- 
ward centrifugal  force. 


110  ELEMENTARY    METEOROLOGY. 

135.  Vorticular  circulation  of  the  atmosphere   around  the  poles.     \\ e 
have  now  to  inquire  whether  the  low  pressure  of  the  atmosphere  around  the 
poles  may  result  directly  or  indirectly  from  the  cause  by  which  the  oblique 
course  of  the  winds  has  been  so  successfully  explained. 

For  this  purpose  the  surface  winds  of  the  temperate  latitudes  and  the  high 
level  currents  above  them,  sidling  swiftly  along  on  their  steep  poleward 
gradients,  must  all  be  considered  together.  They  combine  to  form  a  vast 
aerial  vortex  or  eddy  around  the  pole.  In  the  northern  hemisphere  this  great 
eddy  is  much  interrupted  by  continental  high  pressure  in  winter  or  low 
pressure  in  summer,  and  by  obstruction  from  mountain  ranges,  as  well  as  by 
irregular  disturbances  of  the  general  circulation  in  the  form  of  storms,  large 
and  small.  In  the  southern  hemisphere  the  circumpolar  eddy  is  much  more 
symmetrically  developed.  What  effect  will  be  produced  on  the  pressure  of 
the  atmosphere  in  high  latitudes  by  the  whirling  of  the  great  body  of  air 
around  the  pole  as  a  center  ? 

136.  Cause  of  low  pressure  around  the  poles.      If  the  explanation  of  Sec- 
tion 134  be  now  applied  to  the  atmosphere,  with  the  supposition  that  there  is 
no  loss  of  velocity  by  friction  or  other  resistances,  it  is  clear  that  an  excessive 
velocity  and  a  still  more  excessive  centrifugal  force  would  be  developed  in  the 
circumpolar  vortices.     It  should  be  noticed  that  the  eastward  motion  of  1,000 
miles  an  hour  that  the  air  has  over  the  equator  is  increased  as  the  overflow 
approaches  the  pole ;  at  latitude  60°,  where  the  distance  from  the  axis  is  half 
what  it  was  at  the  equator,  the  eastward  velocity  has  doubled  ;  that  is,  it 
has  become  2,000  miles  an  hour,  or  1,500  miles  faster  eastward  than  the  earth's 
surface  at  that    latitude.     Forty   miles  from  the  pole,  it  would  be  100,000 
miles  an  hour;  and   so  tremendous  a  velocity  on  so  short  a  radius   would 
su  trice   to   hold   the   air   away   from   a   closer    approach   to  the    pole,   if    it 
could,  indeed,  approach  so  close  as  this ;  at  any  less  distance  there  would  be 
a  vacuum. 

But  the  action  of  friction  and  other  resistances  cannot  be  neglected.  The 
presence  of  almost  as  great  an  atmospheric  pressure  in  the  polar  regions  as  at 
the  equator  assures  us  that  the  imaginary  case  of  no  friction  is  lar  from  the 
actual  case.  Although  the  resistances  suffered  by  the  upper  air  currents 
cannot  be  great,  they  successfully  prevent  the  realization  of  the  enormous 
circumpolar  velocities  that  would  result  in  the  case  of  no  friction  and  no 
intermingling  of  currents.  The  reason  for  this  is  seen  in  considering  a^ain 
the  conditions  of  steady  motion  represented  in  Fig.  32.  The  overflow  takes 
so  nearly  an  eastward  direction  that  it  must  travel  over  a  very  long  distance 
in  advancing  from  the  equator  to  the  pole ;  and  in  all  this  distance,  the 
theoretical  increase  nf  velocity  with  decrease  of  radius  is  continually  defeated 
by  the  action  of  the  small  resistances. 


THE    PRESSURE    AND   CIRCULATION    OF    THE    ATMOSPHERE.  Ill 

Excessive  velocities  cannot  be  reached ;  but  from  observations  on  high-level 
clouds,  it  is  known  that  the  currents  five  or  six  miles  above  sea-level  frequently 
move  to  the  eastward  at  a  rate  of  a  hundred  or  more  miles  an  hour ;  it  is 
very  probable  that  much  greater  velocities  would  be  encountered  at  heights 
of  ten  or  fifteen  miles.  Although  the  mass  of  the  atmosphere  may  be  divided 
into  a  lower  and  an  upper  half  at  a  height  of  about  three  miles,  it  is  calculated 
that  half  of  the  capacity  of  all  the  atmospheric  currents  for  doing  work  —  a 
function  of  their  volume,  density  and  velocity  —  is  not  measured  until  a  height 
of  about  eight  miles  is  reached  ;  the  great  velocities  of  the  still  higher  but  much 
smaller  mass  of  air  giving  it  a  capacity  for  work  equal  to  that  of  the  slower 
moving  but  much  greater  mass  below  this  height.  When  it  is  remembered 
that  the  eastward  velocity  is  in  excess  of  the  already  rapid  eastward  movement 
of  the  earth's  surface,  it  will  be  seen  that  the  deflection  towards  the  equator 
arising  from  the  abnormal  centrifugal  force  thus  developed  may  be  fairly 
accepted  as  the  cause  of  the  unexpected  low  pressure  around  the  poles. 

On  a  rotating  earth,  the  convectional  circulation  between  the  equator  and 
the  poles  cannot  follow  the  meridians.  It  must  suffer  deflection  into  oblique 
paths  and  thus  develop  a  vorticular  whirl  around  the  poles. 

The  high  pressure  that  should  result  from  the  low  polar  temperatures  is 
therefore  reversed  into  low  pressure  by  the  excessive  equatorward  centrifugal 
force  of  the  great  circumpolar  whirl ;  and  the  air  thus  held  away  from  the 
polar  regions  is  seen  in  the  tropical  belts  of  high  pressure.  Thus  the  second 
deficiency  of  the  convectional  theory  of  atmospheric  circulation  is  accounted 
for  as  satisfactorily  as  the  first.  £he  theory  may  be  regarded  as  firmly 
established. 

The  credit  of  first  explaining  the  greater  movements  of  the  atmosphere 
and  the  general  distribution  of  pressure  in  accordance  with  just  physical 
principles  belongs  to  Ferrel.  Following  the  results  first  reached  by  him, 
others  have  since  then  confirmed  his  chief  conclusions  and  extended  their 
researches  to  a  fuller  statement  of  meteorological  problems.  Chief  among 
these  later  investigators  may  be  mentioned  Oberbeck,  whose  studies  have  been 
concerned  with  the  general  movements  of  the  atmosphere ;  and  Helmholtz, 
who  has  shown  how  the  general  movements  may  provoke  subordinate  move- 
ments, in  the  manner  briefly  referred  to  in  sections  203  and  237. 


112 


ELEMENT  A 11 V    MET  EO  HOLOG  Y. 


CHAPTER    VII. 


A    GENERAL    CLASSIFICATION    OF    THE    WINDS. 

137.  Basis  of  classification.    Having  now  come  to  a  general  understanding 
of  the  convectional  circulation  of  the  atmosphere  on  the  rotating  earth,  a 
systematic  account  of  the  different  members  of  the  circulation  may  be  attempted. 
A  classification  proposed  by  Dove,  an  eminent  German  meteorologist  of  the 
first  half  of  this  century,  divided  the  prevailing  winds  of  the  world  into  three 
classes  :   the  permanent  winds,  of  which  the  trades  are  the  chief  members  ; 
the  periodical  winds,  of  which  the  monsoons  of  India  are  the  type  ;  and  the 
variable   winds,  comprising  the  irregular  but  prevailing  westerly  winds  of 
middle  latitudes.     While  this  classification  has  been  generally  adopted,  it  does 
not  satisfy  the  present  demands  of  our  science,  being  both  arbitrary  and 
incomplete,  and  failing  to  recognize  the  natural  relation  that  exists  among  the 
various  movements  of  the  atmosphere.     A  classification  according  to  cause  is 
here  presented  in  preference.1     It  is  true  that  we  ordinarily  take  no  account 
of  the  difference  of  cause  between  a  gentle  breeze  of  mild  temperature  and  a 
violent  winter  gale,  preferring  to  classify  them  according  to  their  direction, 
velocity  or  temperature  ;  but  in  seeking  for  an  explanation  of  the  phenomena 
of  the  atmosphere,  it  is  advisable  to  consider  the  various  causes  of  motion 
separately.      The    classification   of   winds   presented  in    the   following  table 
therefore  is  arranged,  first,  according  to  the  source  of  the  energy  on  which 
they  depend,  and  second,  on  the  manner  or  period  of  its  application. 

138.  Classification  of  winds  according  to  cause. 


SOURCE  OF  EXEKOV. 

Al'I'LICATIo.V. 

PEKIOD. 

NAME  OF  WIND. 

Solar  heat     .     .     . 

it       it 
it       a 
it       tt 
tt       tt 
tt       tt 
tt       tt 
tt       tt 

Equator  and  poles      .     . 
Heat  equator  and  poles  . 
Continents  and  oceans    . 
Land  and  water     .     .     . 
Mountains  and  valleys    . 
Local,  or  indirect  . 
Li-ht  and  shadow  .     .     . 
Indirect 

Permanent 
Annual  . 
Annual  . 
Diurnal  .     . 
Diurnal  .     . 
Irregular     . 
Irregular     . 
Accidental  . 

Planetary. 
Terrestrial. 
Continental. 
Land  and  sea  breezes. 

Mountain  and  valley  breezes' 
Cyclonic  and  other  storms 
Kclipsc  winds. 
I  .andslide  and  avalanche  blasts 

•  Lunar  attraction    . 
Telluric  heat      .     . 

Through  the  tides  .     .     . 
Volcanic  eruptions     .     . 

Twice   in  a 
lunar  day  . 

Irregular     . 

Tidal  breezes. 
Volcanic  storms. 

1  Following  closely  the  classification  published  by  the  author  in  the  American  Meteoro- 
logical Journal  for  March,  1888. 


A    GENERAL    CLASSIFICATION    OF    THE    WINDS.  113 

139.  Tidal  breezes.     The  insignificance  of  all  winds  other  than  those  of 
direct  solar  origin  will  be  perceived  by  glancing  at  those  here  referred  to 
lunar,  telluric  and  indirect  solar  causes.     Under  lunar  winds  may  be  placed 
those  light  movements  of  the  air  which  are  thought  to  correspond  with  and 
appear  to  be  determined  by  the  tides,  where  the  rise  and  fall  of  the  sea-surface, 
as  in  estuaries,  is  of  considerable  amount.     The  air  is  raised  and  pushed  away 
by  the  rising  water  ;  and  when  the  water  sinks,  the  air  is  drawn  down  after  it. 
Near  the  borders  of  an  estuary  with  strong  tides,  it  is  said  that  winds  of  this 
class  are  perceptible.     They  are  said  to  occur  in  the  Gulf  of  St.  Lawrence, 
but  they  are  of  subordinate  quality  and  are  generally  overcome  by  stronger 
winds  of  other  kinds.     The  strong  tides  of-  the  Bay  of  Fundy  should  produce 
winds  of  this  class,  if  they  occur  anywhere  :  they  would  be  detected  by  hourly 
records  of  the  wind  direction  at  several  stations  around  the  Bay,  tabulated 
according  to  the  lunar  day,  instead  of  the  solar.     It  is  believed  that  land  and 
sea  breezes  are  intensified  at  certain  tropical  stations,  when  they  move  with 
the  falling  and  rising  tide.     It  may  be  here  mentioned  that  the  most  careful 
analyses  of  wind  records  at  inland  stations,  or  over  the  world  in  general,  have 
failed  to  detect  anything  more  than  the  faintest  and  most  questionable  control 
of  the  winds  by  the  moon,  except  in  the  indirect  manner  above  indicated. 
The  more  closely  the  subject  is  investigated,  the  less  reason  there  appears  to 
be  in  the  popular  belief  that  the  moon  exercises  any  significant  control  over 
the  weather. 

140.  Volcanic  and  accidental  winds.     Winds  are  occasionally  associated 
with  volcanic  outbursts  ;  either  from  the  explosive  action  of  the  eruption  or 
from  the  convectional  motion  of  the  air  over  incandescent  lavas.     These  are 
the  only  winds  of  purely  telluric  origin,  and  need  only  to  be  mentioned  to 
show  their  rarity. 

Destructive  blasts  of  air  are  sometimes  brushed  forward  by  landslides  or 
avalanches ;  they  may  be  of  sufficient  violence  to  overturn  houses  and  trees 
many  hundred  feet  in  advance  of  the  slide.  Although  of  telluric  origin  at 
first  sight,  landslide  blasts  should  be  regarded  as  of  indirect  solar  origin, 
inasmuch  as  landslides  in  all  cases  depend  on  the  erosive  action  of  rain  and 
streams,  and  these  depend  on  sunshine.  Avalanche  blasts  are  more  apparently 
of  indirect  solar  origin. 

Certain  observers  have  reported  a  light  wind  moving  from  the  space 
traversed  by  the  passing  shadow  of  the  moon  during  a  solar  eclipse,  as  if  the 
air  under  the  shadow  became  somewhat  cooled  by  radiation,  and  thus  developed 
a  faint  convectional  descent  and  outflow.  In  the  event  of  a  total  solar  eclipse 
occurring  in  a  populous  country  and  at  a  time  of  quiet  weather,  hourly 
observations  of  the  wind  might  be  undertaken  by  numerous  observers  to 
determine  the  extent  of  this  peculiar  member  of  the  family  of  winds. 


114  ELEMENTARY  METEOROLOGY. 

All  the  lunar,  volcanic,  accidental  and  eclipse  winds  together  are  of  the 
most  trifling  value  compared  with  the  vast  systems  of  prevailing  winds 
embracing  the  whole  earth  in  their  circuits  ;  of  continental  winds,  sweeping  in 
to  and  out  from  the  center  of  even  the  largest  land  areas  ;  or  of  stormy  winds, 
whirling  in  cyclonic  eddies  a  thousand  miles  in  diameter,  travelling  for  a  week 
or  a  fortnight,  and  crossing  lands  and  oceans  on  their  way.  These  are  all 
winds  of  solar  origin. 

141.  Planetary  winds.  All  rotating  planets  that  have  an  atmosphere  and 
are  warmed  around  the  equator  by  a  sun  must  possess  more  or  less  perfectly  an 
oblique  circulation  between  the  equator  and  poles,  of  a  kind  already  outlined 
in  previous  sections,  "the  essential  features  of  such  a  circulation  may  now  bo 
concisely  stated. 

Supposing  the  surface  of  the  planet  to  be  smooth,  and  the  contrast  of 
temperature  between  equator  and  poles  to  be  strong,  there  will  be  a  belt  of 
low  pressure  around  the  equator,  tropical  belts  of  high  pressure  at  some  inter- 
mediate latitude,  and  caps  of  low  pressure  over  the  poles.  The  arrangement 
of  the  isobaric  surfaces  of  the  .atmosphere  would  correspond  to  that  for  the 
southern  hemisphere  in  Fig.  26.  The  overflow  from  the  warm  equator  would 
soon  turn  forward  in  the  direction  of  the  planet's  rotation  ("  eastward")  and 
thus  develop  a  rapid  whirl  around  either  cold  pole.  On  account  of  the 
convergence  of  the  meridians  towards  the  poles,  much  of  the  air  that  departed 
from  the  equator  would  return  in  an  under-current  at  various  intermediate 
latitudes  ;  only  the  smaller  share  would  complete  the  entire  circuit.1  As  the 
branches  from  the  overflow  descend  to  lower  levels  to  begin  their  return 
course,  they  encounter  gradients  still  directed  towards  the  poles,  but  less  steep 
than  those  aloft.  The  strong  equatorward  deflective  force  gained  by  the 
currents  on  the  steeper  upper  gradients  serves  to  carry  them  against  the  slope 
of  the  weaker  lower  gradients  ;  thus  they  return  obliquely  towards  the  equator. 
I  Jut  if  the  currents  descend  close  to  sea-level,  their  velocity  is  so  much  reduced 
by  friction  that  they  obey  the  gradients  and  run  obliquely  towards  the  polos, 
forming  the  prevailing  westerly  winds  of  middle  and  higher  latitudes. - 

1  The  tropical  belts  of  high  pressure  are  sometimes  explained  as  a  result  of  the  crowding 
of  the  equatorial  overflow  as  it  advances  along  the  converging  meridians.     This  is  incorrect. 
If  the  convergence  of  the  meridians  determined  an  increase  of  pressure,  the  pressure  should 
be  highest  at  the  poles  where  the  meridians  converge  most  rapidly.     The  convergence  of  the 
meridians  has  no  significant  share    in  the  increase  of  pressure  towards  the  pole.     Tin-  tropical 
belts  are  due  essentially  to  the  hL'h  temperature  on  the  equatorial  side  and  the  deflective 
force  of  the  circumpolar  whirl  on  the  polar  side. 

2  An  illustration  from  another  point,  of  view  may  make  this  matter  clearer.      In  saying. 
that  the  return  currents  go  back  to  the  equator  against    the  gradients,  it  miuht   he  asked  : 
what  is  the  standard  of  "level"  from  which  the  slope  of  an  isobaric  surface  is  determined'.'- 
The  standard  to  which  we  are  accustomed  is  the  surface  of  the  sea  ;  but  it  must  b<-  remrmberech 


A    GENERAL    CLASSIFICATION    OF    THE    WINDS. 


115 


142.  The  members  of  the  planetary  system  of  winds.  This  theory  of 
the  planetary  circulation  therefore  demands  the  existence  of  an  overflow, 
approaching  the  pole  in  a  spiral  course;  an  intermediate  return  current,  sup- 
plied at  all  latitudes  by  branch  currents  descending  in  short  circuits  from  the 
lofty  overflow  and  receding  from  the  pole  in  a  spiral  course,  but  still  moving 

that  this  surface  is  thirteen  miles  further  from  the  center  of  the  earth  at  the  equator  than  at 
the  poles,  and  that  it  might  therefore  be  said  in  a  certain  sense  to  ascend  from  poles  to 
equator.  The  reason  that  we  call  it  level  is  that  we  are  guided  in  the  determination  of  a 
level  surface  only  by  our  knowledge  of  the  direction  of  gravity,  to  which  a  level  surface  must 
be  perpendicular.  Gravity,  however,  is  not  directed  towards  the  center  of  the  earth,  as  has 
been  explained  in  Section  13.  In  the  youth  of  the  world,  when  its  rotation  was  presumably 
faster  than  now,  a  different  idea  of  "  level "  must  have  obtained. 

For  the  same  reason,  the  idea  of  "level"  that  the  Circumpolar  Eddy  possesses  cannot, 
agree  with  ours ;  for  its  period  of  rotation  around  the  earth's  axis  is  less  than  twenty-four- 
hours.     From  the  Eddy's  point  of  view,  the  surface  of  the  ocean,  which  we  call  level,  must- 
slant  down  towards  the  equator.     Indeed,  the  isobaric  surfaces  at  the  level  of  the  return  - 
current,  which  we  say  slant  to  the  pole,  must  seem  to  the  Eddy  to  slant  faintly  towards  the 
equator ;  and  with  this  understanding  of  gradients,  it  is  natural  that  the  return  current  should 
follow  what  it  regards  as  their  direction  of  descent. 

Figure  33,  corresponding  to  Fig.  26  of  Section  111,  may  make  this  still  plainer.  The-- 
meridional  components  of  the  winds  are  shown  by  dotted  lines  for  a  quadrant  of  the  section.  • 
To  us,  who  live  on  the  surface  of  the  rotating  earth,  the  elliptical  meridian,  -ZVQS,  appears- 
"  level."  If  the  earth  did  not 
rotate,  this  meridian  would 
be  called  "up  hill"  toward 
the  equator,  because  it  rises 
in  that  direction  above  the 
circular  meridian,  nqs, 
which  would  then  be  called 
level.  The  rapidly  whirling 
equatorial  overflow  regards 
all  the  isobaric  surfaces  above 
the  line,  ABC,  as  slanting 
towards  the  poles,  for  in  spite 
of  its  excessive  equator  ward 
deflection,  the  upper  surfaces 
descend  so  rapidly  towards 
the  pole  that  their  descent 
must  be  recognized.  But 
when  the  branches  of  the 
lofty  overflow  descend  to 
enter  the  return  current, 
beneath  A  B  C,  they  find 
isobaric  surfaces  that  are  not 

» •  st«-cp  towards  the  poles  ;  and  these  they  mistake  (as  we  should  say)  for  gradients  directed 
towards  the  equator.  The  line,  A  B  C,  therefore  represents  the  "neutral  plane"  of  the 
planetary  circulation.  Finally,  the  surface  winds,  having  only  a  moderate  velocity  eastward 
in  excess  of  the  earth,  see  the  lower  gradients  as  we  do,  and  sidle  along  them  towards  the 
•  pole. 


FIG.  33. 


116 


ELEMENTARY    METEOROLOGY. 


forward  in  the  direction  of  the  planet's  rotation  until  it  passes  the  axis  of  the 
tropical  high-pressure  belt,  where  it  moves  obliquely  backward  as  the  trade 
wind ;  and  a  lower  current,  constituting  the  prevailing  westerly  winds  of  the 
middle  and  higher  latitudes,  approaching  the  pole  in  a  spiral  course,  like  the 
overflow  aloft.  The  intermediate  and  upper  members  are  not  yet  clearly  dis- 
tinguished by  direct  observation,  for  the  actual  circulation  of  the  earth's 
atmosphere  is  much  disturbed  by  continental  obstruction  and  by  stormy  over- 
turnings.  Indeed,  there  is  reason  to  believe  that  the  confusion  of  currents 
thus  introduced  constitutes  the  greater  part  of  the  resistances  encountered  by 
the  lofty  currents  of  the  equatorial  overflow. 

Calms  should  occur  at  all  places  of  no  gradient ;  that  is,  along  the  axes  of 
the  equatorial  and  tropical  belts  and  close  about  the  two  poles.  The  two 
former  are  well  confirmed  by  observation,  and  record  of  the  two  latter  may  be 
expected  when  the  poles  are  explored. 

Along  the  equator,  above  the  surface  calms,  the  trade  winds  converge  and 
move  from  east  to  west,  ascending  obliquely  as  they  go,  and  gradually  turning 

north  or  south  when  they  mount  to 
an  altitude  at  which  the  poleward 
gradients  appear.  The  overflow  there- 
fore begins  with  a  westward  component 
not  before  mentioned ;  but  this  is  soon 
lost  under  the  action  of  the  deflecting 
force  proper  to  the  hemisphere  that 
the  air  enters  ;  and  the  current  swings 
around  toward  the  east. 

An  ideal  planetary  circulation  is 
represented  in  Fig.  34.  The  upper 
currents  are  drawn  in  full  lines  in  the 
northern  hemisphere,  with  the  inter- 
mediate return  currents  in  dotted  lines. 
The  return  currents  are  drawn  in  full  lines  in  the  southern  hemisphere,  with 
the  surface  winds  in  turn  dotted  beneath  them. 

An  essential  characteristic  of  the  oblique  planetary  circulation  is  the 
retardation  of  the  atmospheric  interchange  between  the  equator  and  the  poles. 
as  compared  with  the  rate  it  would  attain  on  a  non-rotating  planet  on  which 
the  winds  would  follow  the  meridians.  For  while  the  ein-imipolar  winds  on  a 
rotating  globe  reach  much  higher  velocities  than  would  be  gained  by  the 
meridional  winds  of  a  stationary  globe,  the  oblique  course  of  the  cirenni  polar 
winds  reduces  their  meridional  components  of  motion  below  the  values  they 
would  have  in  the  direct  north  and  south  circulation  of  a  stationary  globe. 
The  contrast  of  polar  and  equatorial  temperatures  is  therefore  greater  on  a. 
rotating  than  on  a  non-rotating  planet  under  similar  supplies  of  insolation. 


FIG.  34. 


A    GENERAL    CLASSIFICATION    OF   THE    WINDS.  117 

The  members  of  the  circulation  of  our  atmosphere,  which  may  be  taken 
as  illustrating  the  planetary  system  of  winds,  may  now  be  described.  It  is 
natural  that  they  are  found  in  best  development  over  the  oceans ;  while  the 
continental  areas  cause  interruptions  which  will  be  considered  further  on. 

143.  The  trade  winds  blow  between  the  tropical  belts  of  high  pressure 
towards  the  equatorial   belt  of   low   pressure;    from   the  northeast  in  this 
hemisphere,  and  from  the  southeast  in  the  other.     They  are  best  observed  on 
the  oceans,  where  they  hold  their  courses  steadily  over  great  areas,  the  most 
regular  winds  of  the  world,  with  a  brisk  velocity  on  gentle  gradients.     They 
occupy  nearly  half  the  earth's  surface,  and  thus  add  to  the  uniformity  of  the 
great  torrid  zone,  already  signalized  by  its  faint  contrasts  of  temperature  and 
its  small  change  of  seasons.     Their  name  comes  from  their  steadiness,  and 
not,  as  the  dictionaries  sometimes  say,  from  their  benefit  to  commerce.     Their 
course  is  so  steady  that  it  has  given  name  to  the  Windward  and  Leeward 
Islands  of  the  Lesser  Antilles.     These  great  streams  of  air  average  two  miles 
or  more  in  depth ;    only  the  higher  mountains  of  their  latitudes  rise  above 
them  into  the  oblique  westerly  currents  or  anti-trades  of  still  higher  levels. 
They  are  seldom  invaded  by  storms  ;  they  bear  only  small  clouds  by  day  and 
at  night  they  are  nearly  cloudless  ;  their  mass  warms  throughout  and  expands 
to  greater  volume  as  they  draw  nearer  the  equator ;    and  they  gather  a  great 
amount  of  vapor  from  the  ocean  surface  as  they  brush  the  waters  along  in 
their  course.     Yet  while  the  winds  at  sea  may  blow  for  days  or  even  for  weeks 
with  slight  variation  in  direction  or  strength,  their  uniformity  as  displayed  on 
our  planet  should  not  be  exaggerated.     There  are  times  when  the  trade  winds 
weaken  or  shift ;   there  are  regions  where  their  steady  course  is  deformed, 
most   notably   about  the  larger  island   groups   of  the   Pacific;    the  Fiji  aiul 
Samoa  or  Navigators  Islands.    At  certain  seasons  in  the  several  oceans  they  are 
invaded  by  revolving  storms  or  cyclones  of  terrific  energy  (Sect.  217).     Near 
the  coasts  the  trades  are  interrupted  by  the  daily  breathing  of  the  land  and 
sea  breezes  ;  they  are  greatly  deflected  by  mountain  ranges  and  continental 
barriers  ;  and  over  the  torrid  lands  they  may  be  entirely  broken  up  for  part  of 
the  year  by  the  strong  seasonal  changes  of  temperature. 

144.  The  doldrums  or  equatorial  calms  lie  between  the  steady  trades  along 
the  barometric  equator  of  no  gradients :  a  belt  of  light  and  variable  winds  and 
frequent  calms,  with  cloudy,  rainy  sky,  accompanied  by  thunder-storms  and 
squalls.     The  air  of  the  moist  trade  winds,  flowing  in  obliquely  from  either 
side,  here  loiters  about,  and  were  it  not  for  the  shouldering  off  .of  the  lofty 
overflow  by  the  expansion  of  the  lower  air,  the  equatorial  low-pressure  belt 
would  soon  be  filled  up.     Sailors  find  that  the  doldrums,  with  their  calm, 
sultry  air,  their  light  and  baffling  breezes  and  frequent  rains,  stand  in  dis- 


118  ELEMENTARY    METEOROLOGY. 

agreeable  contrast  with  the  refreshing  air  of  the  trades.  In  the  doldrums  the 
ocean  may  be  glassy  smooth,  reflecting  the  gray  or  leaden  color  of  the  clouds  ; 
the  sails  hang  lazily  from  the  spars,  waiting  for  a  chance  breeze,  flapping  only 
as  the  vessel  rolls  in  the  long  swells  that  swing  across  the  sea  from  stormier 
latitudes.  In  the  trades  the  sea  is  roughened  under  the  brisk  wind  ;  its  color 
is  clear  dark  blue;  ships  sweep  swiftly  across  it,  holding  close  to  their  course, 
every  sail  full  and  drawing,  and  the  vessel  steadily  canted  to  leeward  day  and 
night. 

145,  Horse  latitudes.     The  vague  tropical  belts  of  high  pressure  on  the 
outer  margins  of  the  trades  are  characterized  by  light,  variable  winds  and 
occasional  calms,  known  at  sea  as  the  horse  latitudes  or  tropical  calms  ;  but 
unlike  the  doldrums  the  weather  here  is  comparatively  clear  and  fresh.     The 
meaning  of  these  marked  contrasts,  to  be  given  in  the  chapter  on  rain,  will  be 
found  to  afford  correlations  of  much  value  in  testing  the  general  theory  of 
atmospheric  circulation. 

146.  Prevailing  westerly  winds.     Outside  of  the  tropical  belts  of  high 
pressure,  all  across  the  temperate  zone  and  even  into  the  frigid  regions,  we 
find  the  prevailing  westerly  winds;  Hie  surface  members  blowing  west-southwest 
in  this  hemisphere,  west-northwest  in  the  other.     As  the  upper  and  lower 
currents  are  here  of  nearly  the  same  direction,  the  westerlies  are  of  greater 
depth  than  the  trades  ;  they  prevail  to  the  summits  of  the  highest  mountains 
and  even  to  the  level  of  the  loftiest  clouds.     They  are  frequently  interrupted 
by  storms,  rare  in  the  trade-winds  of  the  torrid  zone  ;  indeed,  the  winter  of  (mi- 
temperate  zone  on  land  is  characterized  by  so  continuous  a  succession  of  shifting 
winds  that  the  prevailing  direction  of  movement  is  hardly  apparent  until  a 
careful  record  is  examined.     The  interference  of  mountains  and  continents 
with  the  westerlies  of  the  northern  hemisphere  is  even  greater  than  with  the 
trades ;  and  for  this  reason,  the  southern  hemisphere,  where  the  temperate 
latitudes  are  nearly  all  of  ocean  surface,  offers  a  much  better  Held  for  the  study 
of  the  westerlies,   as  members  of   the  planetary  circulation,   than  is   found 
nearer  home. 

Between  latitudes  40°  and  60°  south,  the  "brave  west  winds  "  blow  almost 
continuously  from  some  westerly  point;  shifting  somewhat  when  compounded 
witli  passing  cyclonic  whirls,  especially  in  the  higher  latitudes,  and  often 
holding  the  strength  of  a  gale  for  days  In^-thcr.  particularly  in  winter;  if 
reversed  to  an  easterly  direction,  the  wind  remains  in  that  quarter  for  but  a, 
brief  time  and  is  seldom  violent.  For  this  reason,  beating  around  Cape  Horn 
to  the  westward  is  dreaded  by  sailors  ;  the  wind  there  is  boisterous  and  stormy, 
the  air  cold  and  wet,  the  sea  nearly  always  nm^li.  Sailing  vessels  from 
England  to  the  Australian  colonies  therefore  take  an  outward  course  by  Cape 


A    GENERAL    CLASSIFICATION    OF    THE    WINDS.  119 

of  Good  Hope,  but  return  across  the  South  Pacific,  passing  Cape  Horn  to  the 
eastward,  thus  carrying  fair  winds  nearly  all  around  the  world. 

In  passing  to  still  higher  latitudes,  about  60°  in  the  northern  hemisphere 
and  somewhat  further  from  the  equator  in  the  southern,  winds  from  a  polar 
source  become  more  common  ;  very  little  is  known  of  them,  and  as  they,  seem 
to  be  due  to  disturbance  in  the  normal  planetary  winds  caused  by  the  lands, 
they  will  be  briefly  referred  to  in  a  later  section. 

1  147.  The  upper  currents  blow  prevailingly  from  the  west  and  with  high 
velocity  in  nearly  all  latitudes.  Observations  of  the  loftiest  clouds  disclose 
their  rapid  movement  and  almost  constant  drift  from  some  westerly  point. 
Temporary  departures  from  this  direction  in  temperate  latitudes  are  always 
associated  with  some  passing  cyclonic  disturbance.  On  Pikes  Peak,  over 
70  per  cent,  of  the  winds  recorded  in  ten  years  of  observation  came  from 
some  westerly  point,  and  36  per  cent,  from  the  southwest.  Even  in  the  trade- 
wind  belt,  the  winds  on  lofty  peaks,  the  carriage  of  volcanic  smoke  and  ashes, 
and  the  drift  of  the  upper  clouds  indicate  as  steady  a  westerly  wind  or 
anti-trade  aloft  as  easterly  below  ;  the  upper  wind  turning  obliquely  from  the 
equator  while  the  lower  wind  approaches,  it ;  and  the  two  together  undoubtedly 
forming  compensating  members  of  the  terrestrial  circulation.  The  anti-trades 
are  felt  on  the  summit  of  Tenerifte  and  on  the  volcanoes  of  the  Hawaiian 
Islands,  while  clouds  are  seen  floating  in  the  trade-winds  below. 

But  close  to  the  equator,  the  upper  currents  are  from  the  east.  This  is 
known  from  occasional  observations  on  cirrus  clouds,  and  better  still  from  a 
peculiar  effect  of  the  great  volcanic  outburst  of  Krakatoa  in  1883  already 
referred  to.  The  dust  and  vapor  blown  out  by  the  volcano  caused  remarkable 
displays  of  sunset  and  sunrise  colors  (Section  71);  these  were  noticed  at  places 
progressively  further  and  further  west  around  the  equator,  encircling  the  world 
in  fifteen  days  ;  thus  determining  a  westward  transportation  of  the  dust  at  the 
rate  of  seventy  miles  an  hour.  After  encircling  the  earth,  similar  brilliant 
sunsets  were  observed  progressively  northward  and  southward  from  the  equator, 
as  if  the  dust  "were  then  gradually  and  broadly  distributed  by  the  poleward 
overflow  of  the  upper  atmosphere. 

148.  Winds  on  other  planets.  The  telescope  reveals  markings  on  some  of 
the  planets  of  our  system  that  are  interpreted  as  cloud  belts,  arranged  by  their 
winds.  The  great  planet  Jupiter  has  a  well-marked  system  of  belts  ;  their 
distinctness  is  doubtless  in  good  part  due  to  his  rapid  rotation  in  nine  hours 
and  fifty-six  minutes,  which  would  develop  a  strong  deflecting  force  and  cause 
a  distinct  "  flattening"  of  his  winds.  The  axis  of  Jupiter  is  so  nearly  at  right 
angles  to  the  plane  of  his  orbit  that  his  wind  system  can  have  little  annual 
variation.  Saturn,  whose  day  is  almost  as  short  as  that  of  Jupiter,  also  has 


120  ELEMENTARY  METEOROLOGY. 

atmospheric  belts  of  lighter  and  darker  color,  indicating  an  atmospheric; 
circulation  ;  his  axis  being  inclined  almost  27  degrees  to  his  orbit,  and  his 
year  being  long,  his  wind  system  must  shift  north  and  south  like  ours.  A 
planet  like  Uranus,  whose  axis  is  thought  to  be  nearly  coincident  with  the 
plane  of  his  orbit,  would,  if  the  heat  of  the  sun  were  sufficient  at  so  great  a 
distance,  have  an  extraordinarily  well-developed  migration  in  his  wind  system: 
during  its  long  spring  and  autumn,  the  circulation  would  be  like  that  of 
Jupiter ;  but  in  its  equally  long  winter  and  summer,  the  pole  of  the  sunny 
hemisphere,  with  continuous  sunshine  for  many  of  our  years,  would  become 
the  center  of  a  great  cyclonic  whirl,  with  an  ascending  instead  of  a  descending 
component  of  motion.  Planetary  winds  thus  appear  to  be  of  different  kinds. 
We  shall  not  fully  appreciate  the  special  features  of  the  winds  of  our  planet 
until  the  peculiarities  by  which  they  are  distinguished  from  the  Jovian  and 
Uranian  winds  are  clearly  perceived. 

149.  Terrestrial  winds.  The  general  scheme  of  planetary  winds  may  now 
be  more  closely  adapted  to  the  earth  by  considering,  first,  the  effect  of  the 
inclination  of  its  axis  to  the  plane  of  its  orbit ;  second,  the  disturbing  effect 
of  its  continents  and  mountain  ranges.  This  section  will  include  only  the 
first  of  these  effects ;  and  the  planetary  winds  thus  modified  will  be  called  the 
terrestrial  winds. 

The  inclination  of  the  earth's  axis  to  the  plane  of  its  orbit  causes  an 
annual  migration  of  the  heat  equator  and  an  annual  variation  in  the  poleward 
temperature  gradients,  as  has  been  explained  fully  in  a  former  chapter.  In 
consequence  of  this  migration  of  the  heat  equator,  the  barometric  equator,  or 
axis  of  low  equatorial  pressure,  must  also  migrate  with  its  belt  of  light  winds 
and  calms,  and  when  its  extreme  northern  or  southern  position  is  assumed, 
the  winds  of  the  winter  hemisphere  must  extend  across  the  geographic  equator 
for  a  little  distance  into  the  summer  hemisphere.  The  effect  of  this  on  the 
course  of  the  trade  winds  is  remarkable.  Consider  the  time  of  the  late 
northern  summer,  when  the  barometric  equator  on  the  Pacific  lies  between 
7°  and  10°  N.  latitude.  The  southeast  trade  wind  of  the  southern  hemisphere 
is  then  led  by  continuous  northward  gradients  across  the  equator ;  but  on 
entering  our  hemisphere,  and  finding  itself  under  the  control  of  a  right-handed 
deflecting  force,  it  swings  around  and  becomes  a  southwest  wind,  occupying  a 
latitude  belt  that  was  traversed  by  the  normal  northeast  trade  wind  six  months 
before.  Although  not  conspicuous,  on  account  of  the  many  irregularities  to 
which  the  course  of  the  winds  is  subject,  the  special  charts  of  the  Pacific 
clearly  recognize  the  existence  of  this  interesting  feature  of  the  terrestrial 
circulation.  The  opposed  winds  of  this  belt  may  be  called  the  terrestrial 
monsoons  of  the  Pacific;  monsoon  being  a  gpnonil  torm  now  applied  to  winds 
whose  direction  is  reversed  once  a  year.  The  equatorial  counter-current  of 


A   GENERAL   CLASSIFICATION    OF    THE    WINDS.  121 

the  Pacific  waters  appears  to  be  chiefly  due  to  the  action  of  this  special  mem- 
ber of  the  terrestrial  circulation.  A  belt  of  similar  terrestrial  monsoons 
occurs  south  of  the  equator  in  the  Indian  ocean.  When  the  terrestrial  mon- 
soons are  aided  by  continental  influences^  alternating  winds  of  much  greater 
extension  and  distinctness  are  developed,  as  those  of  the  North  Indian  ocean 
and  the  Chinese  seas  (Sect.  153). 

The  annual  change  in  the  value  of  the  poleward  temperature  gradient 
causes  a  corresponding  variation  in  the  barometric  gradient ;  and  although  this 
is  of  small  amount,  it  has  been  recognized  as  a  characteristic  of  the  distribu- 
tion of  pressure  on  the  isobaric  charts  (Sect.  114).  In  consequence  of  this, 
there  is  a  distinct  annual  variation  in  the  velocity  of  the  prevailing  westerly 
surface  winds  of  middle  and  higher  latitudes,  and  of  the  higher  currents  also, 
as  determined  at  mountain  observatories  and  by  observations  of  clouds. 
Extended  cloud  observations  at  Blue  Hill,  Mass.,  indicate  that  the  entire 
atmosphere,  from  the  lowest  to  the  highest  cloud  level,  moves  almost  twice  as 
fast  in  winter  as  in  summer ;  the  mean  velocity  of  the  highest  clouds  in 
winter  being  over  112  miles  an  hour,  and  the  highest  velocity  determined 
being  230  miles  an  hour.  The  velocities  Determined  by  observations  in  Europe 
;uv  not  so  great,  but  they  show  a  similar  annual  variation.  With  increase 
of  velocity  in  the  winds,  the  equatorward  deflecting  force  is  increased  -9  thus 
the  air  is  held  more  effectively  from  the  polar  regions,  and  the  tropical  belt 
of  high  pressure  in  the  winter  hemisphere  is  driven  further  towards  the 
equator  after  the  retreating  doldrums.  When  the  winds  slacken,  the  deflecting 
force  is  relaxed  and  the  high-pressure  belt  moves  towards  the  pole.  The 
"  horse  latitudes  "  therefore  shift  north  and  south  in  an  annual  period  sympa- 
thetically with  the  equatorial  calms  or  doldrums.  This  will  be  found  to  exert 
a  marked  control  on  the  rainy  seasons  of  certain  regions  (Sect.  302). 

150.  Annual  migration  of  the  wind  system.  The  whole  system  of 
surface  winds  shifts  north  and  south  after  the  sun,  moving  northward  till 
August  or  September  and  southward  till  February  or  March,  as  is  shown  in 
the  following  table  of  limiting  latitudes  of  the  several  members  on  the  oceans. 

ATLANTIC  OCEAN.                                           PACIFIC  OCEAN. 

MARCH.  SEPTEMBER.                    MARCH.  SEPTEMBER. 

NE.  Trades  ...  26°  N.  -    3°N.  35°  N.  —  11°N.  25°  N.  -    5°N.  30°  N.  -KPN. 

Doldrums.  ...     3°N.  -    0°  11°N.  -    3°N.  5°N.  —    3°  N.  10°  N.  —    7°N. 

SE.  Trades  ...     0°  N.  -  25°  S.  3°  N.  -  25°  S.  3°  N.  -  28°  S.  7°  N.  —  20°  S. 

The  shifting  is  much  less  than  the  migration  of  the  sun  from  the  equator ; 
and  the  maximum  migration  of  the  wind  system,  north  and  south,  occurs  one 
or  two  months  after  the  solstices ;  an  example  of  the  belated  occurrence  of 
an  effect  after  its  cause. 


122 


ELEMENTARY   METEOROL4  K  J  Y. 


151  Sub-equatorial  and  sub-tropical  wind  belts.  In  consequence  of  the 
shifting  of  the  members  of  the  planetary  system  that  is  seen  in  fine  terrestrial 
winds,  it  is  advisable  to  introduce  special  names  for  those  belts  over  which 

the  equatorial  and  tropical  calm 
belts  annually  migrate.  The  first 
may  be  called  the  sub-equatorial 
belt;  as  has  already  been  stated, 
and  as  is  now  illustrated  in  Fig. 

f*  /  ~~y*'J™farv''J/"~"f~^~f'  35^  its  northern  half  has  alternate 

northeast  and  southwest  mon 
soon  winds ;  its  southern  half, 
southeast  and  northwest  mon- 
soons in  the  winter  and  summer 
of  the  respective  hemispheres. 
The  other  belts  are  called  the 
northern  and  southern  sub-tropi- 
cal belts,  where  the  steady  trades 
and  the  stormy  westerlies  alter- 
nately hold  possession,  summer 
and  winter.  It  must  be  care 
fully  noted  that  these  smaller 
features  of  the  terrestrial  circu- 
lation are  by  no  means  regularly 
and  symmetrically  developed  on 
the  actual  earth,  however  well 
they  might  be  formed  on  an  earth  all  covered  by  an  ocean.  Their  boundaries 
are  not  on  well-defined  latitude  lines,  as  in  the  diagram,  but  are  often  rendered 
vague  and  irregular  by  other  than  terrestrial  causes ;  yet  they  are  distinct  in 
certain  regions,  and  they  will  have  repeated  mention  in  later  sections  concern- 
ing storms,  rainfall  and  climate. 

152.  Continental  winds.  The  interruptions  in  the  terrestrial  winds  due 
to  the  action  of  the  continents  are  of  two  kinds.  One  arises  from  the 
contrasts  of  temperature  on  land  and  water  and  acts  in  opposite  directions, 
winter  and  summer :  it  would  appear  in  full  development,  even  if  the  hinds 
were  perfectly  level  and  elevated  above  the  sea  only  enough  to  prevent. 
their  submergence.  The  other  depends  on  the  obstruction  offered  to  the 
terrestrial  winds  by  the  inequalities  of  the  land,  notably  by  the  plateaus 
and  mountain  ranges:  it  always  acts  in  the  same  way,  and  is  analogous 
to  the  effect  produced  bv  the  continents  mi  the  em-rents  of  the  ocean:  it 
would  appear,  even  if  there  were  no  differences  of  temperature  between  land 
and  water. 


FIG.  35 


A   GENERAL   CLASSIFICATION    OF   THE    WINDS. 


The  isothermal  charts  for  January  and  July  have  already  shown  that  the 
lands  of  the  temperate  zone  are  alternately  warmer  and  colder  than  .the 
adjacent  oceans.  They  must  therefore  cause  seasonal  changes  from  low  to 
high  pressure,1  and  the  terrestrial  winds  must  be  more  or  less  affected  by 
these  changes,  as  has  already  been  briefly  referred  to  in  considering  the  points 
of  correspondence  between  theory  and  observation.  The  inflow  towards  the 
warm  lands  of  summer  and  the  outflow  from  the  cold  lands  of  winter  will  be 
appropriately  deflected  to  the  right  or  left  according  to  the  hemisphere.  The 
overgrown  continent  of  Asia  presents  the  most  striking  illustration  of  winds 
of  this  class ;  the  pressure  over  a  large  part  of  Central  Asia,  when  reduced  to 
sea-level,  varies  by  eight-tenths  of  an  inch  from  January  to  July,  and  the 
general  course  of  the  planetary  winds  is  therefore  greatly  modified,  as  may  be 
seen  on  Charts  V  and  VI.  In  our  western  interior  the  annual  variation  is  about 
four-tenths  of  an  inch  :  in  Australia  it  is  about  three-tenths  of  an  inch.  With 
these  changes  of  pressure  there  are  sometimes  complete  reversals  in  the 
direction  of  the  winds,  which  are  then  called  continental  monsoons. 

A  small  illustration  of  continental  winds  is  found  in  the  peninsula  of 
Spain.  The  seasonal  variations  of  temperature  on  the  peninsula  have  already 
been  shown  in  Figs.  14  and  15,  Section  85.  The  effect  of  these  changes  of 


FIG.  36  (January). 


FIG.  37  (July). 


temperature  on  the  distribution  of  pressure  and  on  the  associated  course  of 
the  winds  is  given  in  Figs.  36  and  37.  The  first  shows  January,  with  an 
interior  area  of  high  pressure  and  obliquely,  outflowing  winds  ;  the  second, 
July,  with  an  interior  area  of  low  pressure  and  obliquely  inflowing  winds. 

153.  The  monsoons  of  Asia.  In  the  winter  season,  when  the  equatorial 
belt  of  low  pressure  lies  eight  or  ten  degrees  south  of  the  geographical  equator 
in  the  Indian  Ocean,  the  abnormally  low  temperatures  prevailing  over  Asia 
increase  the  extent  and  value  of  the  baric  gradients  on  which  the  trade  winds 


1  See  foot-note  to  Sect.  114. 


ELEMENTAUY    METEOROLOGY. 


FIG.  38.  —  GENERAL  WINDS  OK  THE  INDIAN  OCEAN  FOR  JANUARY  AND  FEBRUARY. 
(From  the  Atlas  of  the  Indian  Ocean  of  the  German  Naval  Observatory.) 


A    GENERAL   CLASSIFICATION    OF    THE    WINDS 


125 


FIG.  3J).  —  GENERAL  WINDS  OF  THE  INDIAN  OCEAN  FOR  JULY  AM>  AUGUST. 
(From  the  Atlas  of  the  Indian  Ocean  of  the  German  Naval  Observatory.) 


1:26  KLK.MKN  TAKY     M  KTK<  >R<  )LO(J  Y. 

of  our  hemisphere  blow  ;  and  all  the  southern  and  eastern  coasts  of  that  vast 
continent  are  swept  over  by  an  outflowing  wind,  generally  from  the  north  or 
northwest  on  the  coast  of  China  and  from  the  northeast  over  India,  but  having 
a  direction  locally  much  influenced  by  the  form  of  the  land,  and  hence  better 
named  the  winter  monsoon  than  by  a  name  indicative  of  its  direction.  As 
drawn  in  Fij*.  38,1  the  winter  monsoon  of  northern  India  Mows  from  the  north- 
west in  the  plain  of  the  Ganges.  Over  the  land,  this  wind  is  generally  weak, 
cool,  and  dry  ;  in  China,  when  reinforced  by  stormy  disturbances,  it  may  become 
strong  and  cold.  Over  the  sea,  it  blows  briskly  and  takes  the  normal  direction 
of  the  northeast  trades,  crossing  the  equator  on  its  way  to  the  belt  of  calms  ; 
but  shortly  after  entering  the  southern  hemisphere,  it  appropriately  turns  to 
the  left  of  the  gradients  and  blows  as  a  northwest  wind,  a  true  terrestrial 
monsoon ;  not  so  steadily  here  as  farther  north,  and  yet  appearing  distinctly 
enough  in  charts  of  the  average  wind  direction  of  these  special  latitudes. 

In  the  summer  season,  Asia  is  the  seat  of  unduly  high  temperatures,  and 
by  the  aid  of  its  numerous  lofty  mountains  and  plateaus  a  great  depth  of 
atmosphere  is  warmed  abnormally.  The  high  pressure  of  winter  is  then 
reversed  into  a  low  pressure,  and  the  winds  blow  inward  from  all  sides,  even 
from  the  South  Indian  and  the  Arctic  oceans.  Over  India,  the  general  direction 
of  this  monsoon  is  from  the  southwest ;  but  it  is  turned  to  the  southeast  on  the 
plain  of  the  Ganges.  On  the  coast  of  China,  it  comes  from  the  south  or  south- 
east. It  is  a  warm,  sultry,  moist  and  raiiiy  wind,  of  decidedly  greater  velocity 
than  the  winter  monsoon. 

The  monsoons  of  India  arc  the  most  famous  winds  of  their  class.  Their 
name  is  derived  from  an  Arabic  word,  meaning  season.  The  belt  of  low 
ure  that  lay  to  the  south  of  the  equator  in  our  winter  migrates  gradually 
northward  in  spring,  and  is  finally  replaced  by  the  formation  of  an  area  of  low 
pressure  over  the  warm  desert  plains  of  India  and  Persia,  even  as  early  as  May. 
A  northward  gradient  then  leads  the  southeast  trades  of  the  South  Indian  ocean 
9  the  equator,  and  on  entering  our  hemisphere  they  swing  around  and 
blow  from  the  southwest,  as  shown  in  Fig.  .'5'.)  ;  thus  presenting  one  of  the  most 
interesting  phenomena  in  the  circulation  of  tlie  atmosphere.  The  northern 
Indian  ocean  and  the  adjacent  seas  are  thus  alternately  swept  over  by  winds 
of  northeast  and  southwest  directions,  and  on  land  these  winds  dominate  tin- 
change  of  the  seasons.  The  true  terrestrial  monsoons  are  seen  on  a  bolt  of 
the  Indian  ocean  close  south  of  the  equator:  l)ein;_r  occupied  by  the  normal 

1  !•':  .   in.  and  41   arc  copied   from  charts  prepared  by  Dr.  W.    Koppcn    for  the 

(iennaii  Naval  observatory  (Ih-iil.whe  Seewarte)  at  Hamburg.  Tin-  first  two  are  taken  from 
tin-  Atlas  of  tin-  Indian  Ocean  :  tin- si-con. I  two.  fnun  tin-  Sailing  I  landbook  of  the  Atlantic 
Ocean.  Lon.i:  wind  arrows  denote  steady  win«l<  :  shorter  UTOWtf,  variable  winds.  Heavy 
wind  arrows  denote  '.:ales  or  stroiii:  winds;  liirht  arrows  moderate  winds;  small  circles 
indicate  calms.  A  fuller  account  of  these  charts  is  iriven  in  the  American  Meteorolo-i. -al 
.Journal,  vols.  IX  and  X. 


A   GENERAL   CLASSIFICATION    OF    THE    WINDS.  127 

southeast  trade  in  our  summer,  and  by  the  deflected  extension  of  the  northeast 
trade  which  turns  northwest  after  it  crosses  the  line  in  our  winter.  The 
monsoons  north  of  the  equator  are  a  product  of  terrestrial  and  continental 
winds  combined. 

154.  Monsoons  of  Australia  and  elsewhere.     Australia,  alternately  too 
warm  and  too  cold  for  its  latitude,  and  hence  with  pressures  alternately  lower 
and  higher  than  those  of  the  surrounding  seas,  possesses  winds  blowing  spirally 
inwards  in  January  and  outwards  in  July.    They  are  not,  however,  in  all  parts 
of  this  land  reversed  directly  enough  to  deserve  the  name  of  monsoons.     In 
January  the  equatorial  belt  of  low  pressure  becomes  confluent  with  the  local 
ami  of  low  pressure  over  Australia,  and  the  outflowing  monsoon  of  the  Chinese 
coasts  crosses  the  equator  and  reaches  Australia  as  a  northwest  monsoon  ;  thus 
repeating  the  illustration  already  afforded  by  India  of  a  continental  deformation 
of  the  terrestrial  wind  system. 

In  the  Atlantic  ocean,  north  of  the  equator  and  adjacent  to  Africa,  there  is 
a  small  area  alternately  swept  over  by  the  northerly  trades  of  winter  and  a 
deflected  extension  of  the  southerly  trades  in  summer,  thus  producing  a  distinct 
monsoon-like  reversal  in  the  direction  of  the  winds  with  the  seasons.  The 
same  change  appears  to  occur  in  equatorial  Africa,  as  the  belt  of  calms  shifts 
north  and  south  after  the  sun.  As  the  wind  there  blows  over  the  land  and 
near  the  equator,  its  deflection  from  the  gradients  is  small,  so  that  in  successive 
halves  of  the  year  it  appears  as  a  nearly  reversed  north  and  south  wind,  and 
hence  of  monsoon  quality.  In  equatorial  South  America  this  change  does  not 
appear  so  distinctly ;  the  winds  of  the  plains  o'f  the  Amazon  coming  chiefly  from 
the  east,  probably  by  reason  of  the  heavy  rainfall  that  occurs  along  the  eastern 
base  of  the  Andes,  towards  which  the  air  is  drawn. 

The  winds  on  the  Texas  coast  of  the  Gulf  of  Mexico  manifest  a  distinct 
monsoon  tendency,  blowing  more  frequently  from  the  south  in  the  summer 
and  from  the  north  in  the  winter ;  but  as  their  direction  is  complicated  by  the 
action  of  passing  cyclones,  they  are  not  steady  and  they  do  not  exhibit  the 
strong  and  persistent  seasonal  contrasts  found  in  the  classic  monsoons  of  India. 
Of  all  these  examples,  that  of  Asia  and  in  particular  of  Southern  Asia  and  the 
adjacent  Indian  ocean  is  the  most  instructive.  The  extension  of  the  southeast 
trade-wind  of  the  Indian  ocean  in  the  northern  summer  as  a  southwest  monsoon 
in  the  area  normally  belonging  to  the  northeast  trade,  and  the  reverse  condition 
in  southern  summer,  are  the  most  remarkable  occurrences  of  the  kind  in  the 
world. 

155.  The  monsoon  influence  on  the  terrestrial  winds.     The  cases  of  Asia, 
and  Australia  already   given   illustrate  the  considerable  deformation  of  the 
normal  system  of  terrestrial  winds  by  continental  influences  ;  the  migration  of 


128  ELEMENTARY  METEOROLOGY. 

certain  members  of  the  terrestrial  system  being  greatly  increased  by  the  action 
of  the  land  in  causing  a  strong  migration  of  the  heat  equator.  In  most  parts 
of  the  world,  however,  the  effects  of  the  variation  of  temperature  on  the  land 
are  not  so  excessive  as  to  reverse  the  direction  of  the  wind,  winter  and  summer, 
but  only  to  modify  its  course.  Thus,  in  the  central  and  eastern  United  States, 
the  prevailing  westerlies  veer  from  south  and  south  west  in  summer  when  the 
continental  interior  is  unduly  warm,  to  west  and  northwest  when  it  is  unduly 
cold.  In  Europe,  where  the  greater  continental  area  lies  to  the  east,  the 
monsoon  effect  is  reversed,  and  there  is  a  change  from  west  or  northwest 
winds  in  summer  to  southwest  winds  in  winter.  As  a  compensation  for  these 
oblique  courses  of  the  surface  winds,  the  upper  currents  over  the  eastern 
United  States,  as  determined  by  the  drifting  of  lofty  clouds,  move  more  from 
the  northwest  in  summer  and  from  the  southwest  in  winter  ;  while  over 
western  Europe  their  movement  is  from  the  southwest  in  summer  and 
northwest  in  winter.  The  contrasted  monsoon  influences  on  the  two  sides  of 
the  North  Atlantic  are  of  great  importance  in  determining  the  climatic  features 
of  the  two  regions.  They  are  particularly  effective  in  increasing  the  annual 
range  of  temperature  011  our  eastern  coast,  and  in  diminishing  it  on  the  western 
coast  of  Europe. 

156,  Continental  obstruction  of  terrestrial  winds.  Besides  introducing 
complicated  variations  of  temperature,  the  continental  masses  obstruct  the, 
free  passage  of  the  winds.  One  of  the  most  manifest  effects  of  the  inequalities 
of  the  land  surface  is  found  in  the  general  decrease  of  the  velocity  of  the 
winds  over  the  land  as  compared  with  that  of  the  winds  at  sea.  The  average 
velocity  of  the  latter  approaches  twenty  miles  an  hour  ;  while  that  of  the 
former  is  not  more  than  half  of  this  value. 

The  interference  with  the  regular  course  of  the  terrestrial  winds  exercised 
by  the  stronger  reliefs  of  the  continents  is  well  seen  in  the  western  hemisphere, 
where  the  Cordilleras  of  North  and  South  America  interrupt  the  free  passage 
of  the  winds  between  the  Pacific  and  the  Atlantic.  The  westerlies  of  the 
North  Pacific  branch  southward  and  join  the  trades  along  the  coast  of 
California  and  Mexico ;  and  similarly  the  trades  of  the  Atlantic  give  forth  a. 
bttinch  that  turns  northward  and  reinforces  the  deficiency  in  the  westerlies  in 
the  Mississippi  basin  east  of  our  Cordilleras.  The  same  thing  may  be  noticed 
in  perhaps  better  development  in  the  southern  hemisphere,  where  the  mean 
height  of  the  mountain  chain  is  greater  than  witli  us.  The  westerlies  of  the 
South  Pacific  give  forth  a  great  branch  that  turns  north  along  the  coast  of 
Chile,  and  joins  the  southeast  trade  off  Peru;  the  southeast  trade  of  the 
South  Atlantic  becomes  an  easterly  and  northeasterly  wind  over  P>ra/il,  and 
swings  around  to  join  the  deficient  westerlies  in  the  southern  Argentine  country. 
These  deviations  from  the  normal  paths  of  the  terrestrial  winds  will  be  found 


A    GENERAL    CLASSIFICATION    OF    THE    WINDS.  129 

of  great  importance  in  determining  the  rainfall  of  tneir  several  regions.  In 
the  eastern  hemisphere  there  are  no  great  north  and  south  mountain  ranges, 
and  the  continental  interference  with  the  terrestrial  winds  is  less  marked. 

157.  The  general  winds.    It  will  be  henceforth  convenient  to  speak  of  the 
combined  terrestrial  and  continental  winds  as  the  general  winds.    One  of  their 
most  interesting  features  is  found  in  the  great  wind  eddies  developed  on  the 
oceans.     If  there  were  no  land  areas  to  break  the  even  water  surface  of  the 
world,  the  trades  and  the  westerlies  of  the  terrestrial  circulation  would  be 
developed  in  the  fullest  simplicity,  with  linear  divisions  along  latitude  circles 
between  the  several  members.    The  nearest  approach  to  this  is  in  the  southern 
hemisphere,  as  already  described.     Moreover,  it  is  in  the  winter  season  of 
either  hemisphere  that  the  linear  division  between  the  members  of  the  disturbed 
terrestrial  system  is  most  distinct ;  in  July  it  is  well  marked  in  the  southern 
hemisphere  ;  in  January  it  is  even  more  distinct,  although  somewhat  irregular 
over  the  lands,  in  the  northern  hemisphere.     But  in  the  summer  season,  when 
the  poleward  gradients  in  the  upper  air  are  weakened  and  the  general  circula- 
tion is  relaxed,  the  tropical  belt  of  high  pressure  is  broken  where  it  crosses 
the  warm  lands,  and  the  air  shouldered  off  from  the  lands  accumulates  over 
the  adjacent  oceans,  especially  in  the  northern  or  land  hemisphere.     Then  the 
linear  division  between  the  trades  and  the  westerlies  disappears,  and  the  two 
members  are  united  in  a  general  outflowing  spiral  eddy  or  "  anticyclonic  whirl " 
around  the  oval  area  of  high  pressure  in  mid-ocean.     This  is  finely  illustrated 
for  the  Atlantic  in  Figs.  40  and  41. 

On  the  other  hand,  in  the  winter  season  the  oceans  of  the  northern 
hemisphere  develop  inflowing  spiral  eddies  or  "  cyclonic  whirls "  in  their 
northern  parts,  where  their  temperature  is  abnormally  high  and  the  thermal 
contrast  with  the  lands  of  the  same  latitude  is  strongly  marked.  At  the  same 
time,  the  strength  of  the  southwesterly  winds  over  the  North  Atlantic  is 
increased,  as  appears  in  Fig.  40. 

158.  Arctic  winds.      Still  another  effect  of  the  lands  in  disturbing  the 
normal  terrestrial  winds  is  found  in  the  Arctic  regions,  of  much  interest  in  its 
theoretical  aspect.     It  will  be  recalled  that  the  low  pressure  in  the  polar 
regions  depends  entirely  on  the  centrifugal  force  developed  in  the  deflected  whirl 
of  the  planetary  winds.     If  the  velocity  of  the  circumpolar  whirl  near  the 
pole  were  sufficiently  reduced  by  continental  obstructions,  the  high  pressure 
due  to  cold  might  not  be  entirely  overcome.     This  appears  to  be  the  case 
around  the  north  pole,  the  center  of  the  hemisphere  in  which  the  land  area 
widens  and  becomes  excessive  in  high  latitudes.     The  polar  low  pressure  here 
is  by  no  means  distinct,  for  the  gradients  are  directed  from  the  polar  region 
in  summer  towards  the  centers  of  low  pressure  on  the  continents,  and  in  winter 
towards  the  low  pressure  areas  of  the  northern  oceans.     In  both  seasons  thero 


I1'  I'..     10.    —    (  ,|.  M.I;  \  |.      \VlMi-.     <>|       I  HI       A  I  I.  \N  I  I'        I  «.!;     .1   \\|     \KV. 

130  (from  the  Atlantic  Sallu,,,   !l<, „,//„,',/.•  ,,/  tl,,    (;,ru>«u    \,,r,il   <»>*,  n-ntnrii.) 


FIG.  41. — GENERAL  WINDS  OF  THE  ATLANTIC  FOR  Jn.Y. 
(From  (he  Atlantic  Sail  in  f/  Handbook  of  the  German   \>n;il  (»,*>rrritory.) 


131 


182  ELEMENTARY   METEOROLOGY. 

are  northerly  or  northeasterly  winds  issuing  from  certain  parts  of  the  region 
around  the  pole ;  these  may  be  regarded  as  lingering  representatives  of  a  vast 
system  of  polar  winds  that  would  sweep  towards  the  equator  if  a  strong  high 
pressure  at  the  poles  had  not  been  so  nearly  reversed  to  low  pressure  by  the 
whirl  of  the  circumpolar  winds.  In  the  southern  hemisphere,  southeast  winds 
are  reported  as  prevalent  in  high  latitudes  :  hence  an  increased  pressure  may 
be  expected  around  the  pole  ;  but  the  reduction  of  pressure  around  the  south 
pole  is  more  strongly  marked  and  the  winds  there  are  stronger  than  in  the 
northern  hemisphere,  even  though  the  poleward  temperature  gradients  in  that 
hemisphere  are  somewhat  weaker  than  in  ours.  This  want  of  symmetry  in 
the  wind  systems  of  the  two  hemispheres  must  be  referred  to  the  unsymmet- 
rical  distribution  of  land  with  respect  to  the  equator. 

159.  Diurnal  variation  in  wind  velocity  on  land.  Over  the  oceans,  the 
velocity  of  the  wind  shows  no  distinct  diurnal  period ;  but  over  the  lands  and 
particularly  in  clear  warm  weather,  the  winds  are  distinctly  stronger  about 
noon  than  in  the  night.  This  is  fully  accounted  for  by  the  convectional 
interchange  in  the  day-time  between  the  surface  air  and  the  faster  moving 
currents  at  a  height  of  one  or  several  thousand  feet,  as  explained  in  Section  54. 
At  night,  the  wind  near  the  surface  of  the  earth  is  greatly  retarded  by  friction, 
and  before  morning  generally  falls  to  a  calm,  unless  urged  by  some  temporary 
stormy  disturbance :  at  a  height  of  a  thousand  feet,  the  movement  of  the  air 
continues  about  as  usual.  Intermediate  layers  of  air  retain  velocities  dependent 
on  the  greater  or  less  frictional  resistances  that  they  suffer.  But  as  soon  as 
the  instability  of  morning  hours  causes  convectional  interchange  between  the 
various  layers,  their  movement  is  more  nearly  equalized  and  hence  the  velocity 
of  the  wind  is  increased  at  the  earth's  surface,  reaching  a  maximum  in  the 
early  afternoon.  As  the  ground  cools  towards  sunset  and  the  convectional 
currents  cease,  the  surface  winds  weaken  and  the  evening  becomes  calm.  It 
will  be  seen  that  the  diurnal  increase  of  cumulus  clouds  (Sect.  196)  is  caused 
by  the  same  convectional  process.  In  fair  summer  weather,  the  variation  in 
the  velocity  of  the  wind  and  in  the  amount  of  cloudiness  is  very  distinct  even 
to  ordinary  observation.  The  change  in  the  velocity  of  the  wind  is  most 
pronounced  in  arid  regions,  such  as  our  drier  western  plains,  where  the  diurnal 
variation  of  temperature  in  the  lower  air  is  strong  :  at  night  the  calm  is  often 
complete ;  in  the  day-time  the  wind  may  rise  to  a  dusty  gale.  The  dust 
whirlwinds  of  desert  plains  should  be  associated  with  this  feature  of  the 
general  winds,  as  they  are  only  the  more  visible  manifestation  of  the  process 
on  which  the  diurnal  increase  in  velocity  generally  depends.  The  greater 
velocity  of  the  summer  than  of  the  winter  monsoon  in  India,  and  in  general 
the  greater  velocity  of  the  wind  on  a  given  gradient  in  summer  than  in  winter, 
is  best  explained  by  this  process. 


A   GENERAL   CLASSIFICATION    OF   THE   WINDS.  133 

It  must  be  inferred  from  this  peculiarity  of  the  general  winds  that  the 
weak  gradients  on  which  they  depend,  illustrated  in  the  isobaric  charts  IV,  V, 
VI,  are  not  sufficient  to  drive  the  lower  air  across  the  rough  surface  of  the 
lands,  unless  aided  by  the  intermixture  of  the  lower  and  higher  layers  by 
convection.  When  winds  occur  at  night  on  land,  they  are  to  be  referred  to 
local  gradients,  such  as  are  described  in  the  following  sections,  or  such  as 
accompany  the  passage  of  storms. 

Furthermore,  according  to  this  theory,  a  necessary  consequence  of  the 
diurnal  variation  in  the  velocity  of  the  surface  wind  over  the  land  is  an 
inverse  variation  in  the  velocity  of  the  upper  wind.  The  velocity  aloft  should 
decrease  by  day,  for  the  gravitative  acceleration  on  the  general  gradients  has 
then  to  expend  a  part  of  its  force  in  overcoming  friction  with  the  ground ; 
and  the  velocity  aloft  should  increase  at  night,  when  the  lower  air  lies  still 
and  the  acceleration  has  to  overcome  only  the  friction  of  air  on  air.  The 
records  of  mountain  observatories  show  very  clearly  that  the  variation  thus 
called  for  «by  theory  really  exists.  Even  at  the  moderate  height  of  the  Eiffel 
tower  (990  feet)  in  Paris,  the  nocturnal  increase  in  the  velocity  of  the  wind  is 
distinct.  The  explanation  as  applied  to  the  lower  winds  is  due  to  Espy,  who 
announced  it  in  1840 ;  the  variation  of  the  upper  winds  was  detected  by 
Hellman  of  Berlin,  in  his  study  of  our  Mt.  Washington  records  in  1875  ;  it 
was  explained  by  Koppen  in  the  same  year. 

The  vertical  interchange  of  upper  and  lower  air  in  the  day-time  has  been 
applied  to  explain  the  hot  noon-time  westerly  winds  of  the  plains  of  northern 
India.  These  winds  have  a  less  amount  of  moisture  than  is  observed  in  the 
region  whence  they  seem  to  come.  It  is  ingeniously  suggested  that  this 
peculiarity  results  from  a  convectional  descent  of  dry  upper  air  from  an 
elevation  of  about  10,000  feet,  to  which  the  surface  heat  would  cause  the 
lower  air  to  ascend.  At  that  level,  the  calculated  gradients  would  cause 
westerly  winds ;  and  the  dry  upper  air,  descending,  would  reach  the  ground 
with  an  abnormal  direction,  a  high  temperature,  and  an  unusually  small 
amount  of  moisture.  A  similar  explanation  may  be  found  to  apply  in  northern 
Texas  and  Kansas  where  parching  hot  westerly  winds  are  known  on  summer 
days  (Sect.  245). 

There  is  a  slight  diurnal  variation  in  the  mean  direction  of  wind,  of  small 
practical  importance,  but  of  much  interest  from  the  evidence  that  it  gives  of 
the  correctness  of  the  principles  in  accordance  with  which  the  theory  of  the 
winds  has  been  framed.  The  average  of  many  hourly  observations  shows  that 
the  wind  tends  to  veer  a  little  to  the  right  in  this  hemisphere  as  the  day 
passes,  and  to  turn  back  again  as  night  comes  on.  This  is  in  consequence  of 
the  convectional  descent  of  the  upper  air,  as  just  explained ;  for  the  upper 
currents,  with  little  friction,  turn  strongly  from  the  gradient,  and  this  effect  is 
propagated  downward  in  the  day-time  towards  the  earth's  surface  ;  but  at  night 


134  ELEMENTAL Y  METEOROLOGY. 

when  surface  friction  with  the  ground  is  more  largely  in  control,  the  deflection 
from  the  gradient  is  less  ;  hence  the  surface  wind  turns  a  little  to  the  right  of 
its  mean  direction  by  day,  and  to  the  left  by  night. 

160,  Land  and  sea  breezes.  The  seasonal  contrasts  of  temperature  on 
land  and  water  have  a  parallel  in  the  diurnal  contrasts.  In  summer  particu- 
larly, the  land  over  its  whole  area  is  warmer  than  the  sea  by  day,  and  cooler 
by  night ;  and  if  our  days  were  long  enough,  diurnal  winds  would  sweep  into 
the  continents  to  their  very  centers  every  day,  and  back  again  every  night. 
But  the  rotation  of  the  earth  is  so  rapid  in  comparison  to  the  circulation 
of  the  winds  excited  by  the  diurnal  contrasts  of  temperature  that  there  is  not 
time  for  their  motion  to  be  propagated  far  inland  or  seaward  from  the  coast- 
line. The  literal  breezes  seldom  extend  more  than  twenty  or  thirty  miles 
inland,  and  their  seaward  extension  is  probably  less.  Delicate  observations 
have  detected  a  slight  diurnal  variation  of  pressure  at  stations  on  the  coast 
and  over  the  neighboring  interior,  similar  to  that  between  the  sea  and  land  in 
winter  and  summer,  but  of  much  less  amount. 

The  sea-breeze  begins  in  the  morning  hours,  from  nine  to  eleven  o'clock,  as 
the  land  warms ;  it  brings  in  the  pure  cool  air  from  the  sea.  In  the  late 
afternoon  it  dies  away,  and  in  the  evening  the  land  breeze  springs  up  and 
blows  gently  out  to  sea  till  morning.  This  process  is  repeated  with  great 
regularity  in  the  tropics,  where  there  are  few  storms  and  the  diurnal  changes 
are  the  chief  ones.  In  our  latitudes,  the  land  and  sea  breezes  are  often  masked 
or  overcome  by  the  winds  of  cyclonic  storms  ;  they  appear  only  in  the  spring 
or  summer,  when  the  air  over  the  land  may  become  for  some  hours  decidedly 
warmer  than  over  the  sea.  The  sea  breeze  reduces  the  temperature  on  its 
arrival  and  prevents  a  high  noon  maximum  ;  a  thermograph  often  exhibits 
an  early  morning  and  a  late  afternoon  maximum,  with  a  moderate  depression 
in  the  curve  between  the  two  during  the  blowing  of  the  breeze,  as  appears  in 
Fig.  11.  It  thus  diminishes  the  range  and  lowers  the  mean  temperature  on 
the  coast.  On  tropical  coasts,  the  Seabreeze  is  the  healthful  wind  ;  the  land 
breeze,  often  blowing  from  miasmatic  swamps,  is  fever-laden,  bearing  the  odors 
of  the  soil  (Sect.  310). 

The  changes  of  pressure  of  diurnal  period  on  the  Iberian  peninsula  may  be 
here  referred  to,  in  continuation  of  the  illustrations  of  annual  variations  of 
temperature  and  pressure  already  afforded  by  that  region.  The  mean  pressure 
for  9  A.M.  for  July,  Fig.  42,  exhibits  a  weaker  system  of  centripetal  gradients 
than  appears  for  the  mean  of  July,  as  given  in  Fig.  37;  but  the  pressure 
for  3  P.M.  for  July,  Fig.  43,  shows  gradients  of  greater  value.  In  this 
case,  we  may  therefore  expect  that  the  occurrence  of  the  literal  sea  breeze 
of  day-time  is  associated  with  an  incn-ase  of  the  general  inflow  of  tin; 
summer  winds  ;  while  in  the  early  morning,  before  the  temperature  has  risen, 


A   GENERAL   CLASSIFICATION    OF    THE    WINDS. 


135 


fche  inflow  must  be  weaker.  A  figure  for  nocturnal  hours  has  not  been  prepared, 
for  lack  of  observations  ;  it  would  probably  show  a  further  weakening  of  the 
inflow,  with  an  off-shore  gradient  around  the  coast. 

The  land  and  sea  breezes  follow  the  local  gradients  closely  when  they 
begin  ;  but  as  they  are  drawn  in  from  a  greater  distance,  .they  manifest  a 
distinct  tendency  to  deflection.  If  the  sea  breeze  is  easterly  at  first,  it  veers 


FIG.  42  (July,  9  A.M.), 


FIG.  43  (July,  3  P.M.). 


to  southerly  before  fading  away ;  then  the  land  breeze,  coming  first  from  the 
west,  veers  toward  the  north  late  at  night.  Thus  a  systematic  diurnal  rotation 
of  the  wind  is  produced ;  such  veering  breezes  are  locally  known  as  "  round- 
abouts "  on  the  coast  of  Massachusetts.  The  same  change  is  well  known 
elsewhere.  It  is  often, said:  we  know  the  earth  turns  around,  because  the 
sun  and  stars  rise  and  set.  It  might  also  be  said :  we  know  the  earth  turns 
around,  because  the  trades  and  the  westerlies  blow  obliquely,  or  because  the 
land  and  sea  breezes  veer  in  regular  order. 

The  average  hourly  direction  of  the  land  and  lake  breeze  at  Chicago  for 
July,  1882,  Fig.  44,  gives  excellent  illustration  of  a  regular  veering  from  the 
lake  breeze  of  afternoon  into  the  land  breeze  at  night,  with  a  sudden  reversal 


"     2       J       45        67        e        9       <0       II     NOON    T1     Z       3        4        5        6        7       6        9      t/o     ^l 


/ 


\ 


FIG.  44. 


of  the  latter  into  the  former  at  noon.  The  sea  breeze  is  felt  earlier  on  low 
ground  than  on  high  ;  the  depth  of  the  breeze  on  our  sea-coast  may  be  roughly 
gauged  from  observations  made  in  a  captive  balloon  at  Coney  Island,  near 
New  York  ;  the  average  elevation  at  which  the  cool  inflow  from  the  sea  was 
exchanged  for  the  overlying  warm  outflow  from  the  land  was  from  five  to  six 
hundred  feet. 


136  ELEMENTARY    METEOROLOGY. 

161,  The  sea  breeze   begins   off-shore.      There  is  one  peculiarity  of  the 
st-a  breeze  that  deserves  mention,  as  it  does  not  appear  in  the  continental 
winds  whose  period  is  so  much  longer  than  twenty-four  hours.    The  sea  breeze, 
as  a  rule,  makes  its  first  appearance  off-shore  and  gradually  beats  its  way  to 
the  land.     It  would  appear  at  first  sight  that  its  growth  should  be  in  just  the 
other  direction  ;  for  when  the  pressure  decreases  over  the  land  by  the  overflow 
of  warmed  air,  there  should  be  an  immediate  response  from  the  adjacent  lower 
air,  which,  it  would  seem,  should  at  once  flow  in  to  the  region  of  diminished 
pressure,  and  then  gradually  extend  its  area  backwards  to  a  greater  and  greater 
distance  from  the  land.     But  such  is  not  the  case  ;  and  the  following  explana- 
tion has  been  offered  by  Seemann  to  account  for  the  observed  facts.     The 
winds  that  have  been  thus  far  explained  are  examples  of  steady  motion,  in 
which  a  condition  of  equilibrium  has  been  attained  between  the  accelerating 
force  of  gravity,  the  deflective  force  of  the  earth's  rotation  and  the  resistance 
of  friction  (Sect.  94).     Let  us  suppose  for  a  moment  that  in  the  case  of  the 
land  and  sea  breeze,  the  change  of  temperature  in  the  air  over  the  land  is 
immediate,  warming  at  once  at  sunrise  and  cooling  as  quickly  at  sunset.    There 
would  manifestly  be  a  time  following  these  abrupt  changes   in  which  the 
circulation  of  the  air  was  not  adjusted  to  the  gradients  offered  to  it.     The 
rapid  expansion  of  the  air  on  the  ground  could  not  wait  for  the  overflow  of 
the  air  from  above  it,  but  would  at  once  demand  and  secure  more  space  for 
itself  by  expanding  laterally  to  seaward,  where  the  air  is  cool  and  its  expansive 
force  relatively  weak.     But  as  the  upper  air  flows  away  and  relieves  the  lower 
air  from  its   constraint,  the  conditions  of  ordinary  convectional  circulation 
gradually  appear.     These  conditions  will  be  established  first  a  short  distance 
from  the  shore  where  the  overflow  from  the  land  accumulates,  and  will  gradu- 
ally make  their  way  to  the  land. 

The  actual  case  has  not  the  immediate  changes  of  temperature  here 
supposed,  but  it  may  be  presumed  that  the  rate  of  warming  of  the  air  on  the 
land  is  for  a  time  in  the  morning  so  rapid  that  the  expansion  of  the  air  keeps 
in  advance  of  the  overflow  by  which  it  is  accommodated  ;  and  as  long  as  the 
rise  of  temperature  thus  causes  an  increase  in  the  gradients,  steady  motion 
cannot  be  reached.  A  reaction,  such  as  the  pressure  of  the  land  air  toward 
the  sea,  must  exist  as  long  as  the  gradients  and  the  rate  of  motion  on  them  are 
increasing;  the  delay  in  the  establishment  of  the  Seabreeze  and  its  iirst 
appearance  in  the  offing  may  be  ascribed  to  this  reaction.  A  reported  increased 
strength  and  high  temperature  of  the  tropical  land-breeze  in  the  morning,  just 
before  the  sea  breeze  sets  in,  is  confirmatory  of  this  theory. 

162.  Combination  of  general  and  litoral  winds.     When  the  changes  of 
temperature  that  would  cause  a  normal  sea  breeze  in  ;i  stationary  atmosphere 
take  place  in  a  wind  of  other  origin,  they  modify  its  direction  or  intensity. 


A    GENERAL   CLASSIFICATION    OF    THE    WINDS.  137 

The  westerly  wind  that  prevails  in  Ohio  in  summer  is  deflected  in  the  neigh- 
borhood of  Lake  Erie  into  a  northwesterly  wind  by  day  and  a  southwesterly 
wind  by  night,  because  of  the  contrasts  of  temperature  on  either  side  of  the 
shore-line,  which  lies  about  parallel  to  the  course  of  the  general  wind.  On 
th*'  northern  coast  of  Long  Island  Sound,  in  fine  summer  weather,  the  wind 
generally  comes  from  the  west,  but  shifts  from  northerly  at  sunrise  to  southerly 
at  noon.  On  the  coasts  of  California  and  Chile,  the  prevailing  west  wind  is 
intensified  in  summer  time  into  a  moderate  gale  by  day  and  retarded  to  a  calm 
or  even  reversed  to  a  light  land  breeze  by  night.  The  east  coasts  in  the  trade- 
wind  belt  show  a  similar  variation  in  the  strength  of  their  winds. 

163.  Mountain  and  valley  breezes.  The  seasonal  and  diurnal  winds  that 
have  been  considered  thus  far  would  appear  en*  a  level  earth.  There  remains 
a  class  of  breezes  of  diurnal  period  whose  opportunity  depends  on  the 
unevenness  of  the  land  surface.  These  are  felt  in  valleys  at  night,  blowing 
down  stream  and  increasing  to  a  brisk  gale  where  a  large  valley  emerges  on  an 
open  plain ;  and  on  the  higher  mountain  sides  by  day,  blowing  up  the  slope, 
but  with  less  velocity  than  is  attained  by  the  concentrated  nocturnal  down- 
flow. 

The  explanation  of  the  nocturnal  mountain  breeze  is  not  far  to  seek.  It 
results  simply  enough  from  the  faster  cooling  of  the  surface  stratum  of  air 
that  has  already  often  been  referred  to ;  when  the  cool  and  therefore  heavy 
stratum  lies  on  a  slope,  it  causes  descending  currents.  If  the  form  of  the 
surface  concentrates  this  aerial  drainage  from  a  large  upland  region  into  a 
narrow  valley  outlet,  a  mountain  breeze  of  some  violence  will  appear  in  its 


lower  course.  The  up-stream  valley  breeze  of  day-time  may  be  explained  as 
follows  :  Let  ABODE,  Fig.  45,  be  the  cross-section  of  a  valley,  in  which  the 
morning  air  lies  quiet,  with  level  isobaric  surfaces,  such  as  AGE.  As  the  day 
advances  and  the  lower  air,  MNR,  warms,  it  expands  and  lifts  the  overlying 
air  bodily.  The  greater  part  of  the  isobar  will  be  evenly  lifted  by  the  amount 
of  the  expansion  in  the  air  near  the  surface  beneath  it,  and  will  be  found  at 
•IKL;  but  towards  the  sides  of  the  valley,  where  the  isobar  runs  near  the 
ground,  the  lifting  that  it  suffers  is  less,  and  at  the  points  A  and  E  its  position 
remains  unchanged.  At  either  side  of  the  valley  faint  gradients,  JA  and  LE, 
are  thus  formed,  on  which  the  air  is  urged  towards  the  slopes,  and  there,  by 
reaction  with  the  ground,  an  up-hill  current  is  formed.  As  in  the  previous 


138  ELEMENTARY    METEOROLOGY. 

case,  it  appears  with  greater  strength  where  the  form  of  the  surface  concen- 
trates its  flow,  and  hence  is  to  be  looked  for  in  lateral  valleys  rather  than  on 
the  spurs  between  them.  As  the  temperature  of  the  ascending  air  is  lowered 
by  expansion,  the  valley  breezes  that  rise  past  a  mountain  peak  may  retard  its 
diurnal  rise  of  temperature,  as  seems  to  appear  in  Fig.  12,  b. 

Fragmentary  accounts  of  such  winds  are  found  in  the  narratives  of  oui 
western  exploring  expeditions,  but  they  have  not  yet  received  the  careful 
description  that  they  deserve.  They  are  doubtless  to  be  met  with  in  all  parts 
of  the  western  mountainous  country  ;  the  deep  valleys  leading  out  from  the 
Sierra  Nevada  into  the  plain  of  California  should  show  the  nocturnal  mountain 
breeze  to  perfection.  In  the  Andes  and  the  Himalaya  they  are  well  known  ; 
in  the  latter  they  are  described  as  blowing  up  the  valleys  by  day  from  nine 
o'clock  in  the  morning  to  an  early  hour  in  the  evening  ;  at  night  they  blow 
down  again,  and  where  the  larger  streams  open  out  to  the  plains,  they  attain 
some  violence  :  but  on  the  high  plateaus  the  nights  are  calm. 

In  elevated  valleys  between  snow-covered  mountains  the  air  lying  on  the 
snowy  slopes  remains  cold  through  the  day,  while  the  air  on  the  valley  ground 
is  warmed  ;  in  such  cases  a  cold,  stormy  descending  current  is  felt  on  the  slopes, 
and  an  ascending  current  may  be  expected  over  the  middle  of  the  valleys. 
The  lateral  descending  current  has  been  observed  on  the  high  snow-fields  of 
the  Andes.  On  the  other  hand,  when  a  valley  is  occupied  by  a  glacier,  the  air 
near  the  ice  is  held  at  a  low  temperature,  and  may,  even  in  day-time,  form 
a  mountain  breeze,  which  is  ordinarily  limited  to  the  night.  Descending 
glacial  breezes  of  this  kind  are  reported  from  the  Muir  glacier  of  Alaska. 

The  presence  of  mountains  near  a  coast-line  where  the  land  and  sea  breezes 
are  felt,  serves  to  intensify  them  by  the  additional  causes  of  motion  then 
introduced.  Even  the  great  monsoon  system  of  Asia  is  aided  in  this  way, 
Asia  being  a  land  of  great  relief. 

164.  Mountain  breezes  and  inversions  of  temperature.  The  nocturnal 
inversions  of  temperature,  described  in  Section  43,  characterize  clear  weather 
on  plains,  where  the  wind  falls  to  a  calm  in  the  evening.  There  can  be  little 
question  that  balloon  ascents  over  plains  during  clear,  quiet  nights  would  in 
nearly  all  cases  discover  an  increase  of  temperature  with  ascent  for  several 
hundred  feet.  This  inversion  <>t  the  vertical  temperature  gradient  has  been 
ascribed  to  the  cooling  of  the  lower  air  by  conduction  and  radiation  to  tin1 
cooled  ground  ;  it  is  independent  of  any  motion  in  the  air. 

In  hilly  regions  the  nocturnal  inversions  of  temperature  are  not  only 
rendered  more  apparent  from  the  ease  with  which  they  may  be  observed  ; 
they  are  intensified  by  a  gentle  aerial  drainage  down  the  slopes,  analogous  to 
tin-  mountain  breezes  above  described,  but  of  much  gentler  motion.  As  night 
comes  on,  the  chilled  air  on  the  hill-tops  and  slopes  runs  down  into  the 


A    GENERAL    CLASSIFICATION    OF    THE    WINDS.  139 

adjacent  valley,  and  there  accumulates  in  much  greater  thickness  than  it 
would  gain  on,  level  ground  ;  while  the  hill-tops  continually  receive  a  .supply 
of  uncooled  air,  settling  on  them  from  above,  and  retain  a  relatively  high 
temperature.  The  marked  contrast  between  the  mild  air  of  midnight  on  the 
hills  and  the  chilly  air  in  the  valleys  may  often  be  observed  even  where  the 
difference  of  level  does  not  reach  a  hundred  feet. 

In  the  case  of  a  strong  mountain  breeze  issuing  from  a  large  valley  out 
upon  a  broad  plain,  a  somewhat  different  result  may  be  expected.  As  long  as 
the  descent  of  the  breeze  is  slow,  the  loss  of  heat  by  conduction  and  radiation 
to  the  ground  may  entirely  overcome  its  tendency  to  an  adiabatic  increase  of 
temperature  by  compression  in  descent.  If  not  counteracted,  this  would  cause 
a  rise  of  one  degree  for  every  188  feet  of  descent.  When  the  descent  is  rapid, 
as  in  a  breeze  draining  a  large  surface  of  mountain  slopes,  all  converging  to 
discharge  by  a  single  valley  outlet,  then  the  gain  of  heat  by  compression  may 
appreciably  decrease  the  cooling  of  the  breeze  ;  and  as  such  a  breeze  issues 
from  the  valley-mouth  to  the  plain,  it  may  be  of  higher  temperature  than  that 
to  which  the  quiet  air  at  the  same  level  on  the  plain  some  distance  away  has 
fallen.  While  this  is  in  a  measure  only  a  theoretical  deduction,  it  receives 
some  confirmation  from  an  account  of  a  western  military  station  at  the  foot  of 
a  mountain  range  where  a  valley  opened  to  an  elevated  plain.  The  height  of 
the  plain  was  too  great  for  the  successful  growth  of  wheat ;  but  near  the  fort, 
by  the  mouth  of  the  valley,  wheat  was  safely  harvested.  Although  the 
observers  there  did  not  mention  the  mountain  breeze  as  the  local  safeguard 
against  nocturnal  frosts,  it  does  not  appear  unlikely  that  such  may  have  been 
the  case. 

165,  Winds  not  yet  classified.  If  the  charts  of  the  average  winds  for 
January  and  July  are  now  examined,  it  will* be  found  that  nearly  all  of  the 
many  directions  that  the  winds  follow  in  one  region  or  another  may  be  explained 
by  reference  to  some  of  the  foregoing  paragraphs.  But  if  our  daily  weather- 
maps  are  examined,  we  may  often  see  south  winds  in  the  Mississippi 
valley  in  winter,  and  north  winds  there  in  summer,  which  find  no  explanation 
from  the  causes  thus  far  considered.  Their  origin  will  be  stated  in  the 
chapter  on  cyclonic  storms  :  but  as  these  storms  are  always  accompanied  by 
clouds  and  rain,  some  account  of  the  moisture  of  the  atmosphere  must  be  next 
introduced. 


ELEMENTARY    METEOROLOGY. 


CHAPTER   VIII. 

THE   MOISTURE   OF   THE   ATMOSPHERE. 

166.  Evaporation.  The  presence  of  a  water  surface  over  three-fourths  ot 
the  earth  insures  a  continual  supply  of  water  vapor  for  the  atmosphere ;  while 
the  interruption  of  the  ocean  surface  by  continents  and  the  great  variations  of 
temperature  in  time  and  place  require  that  the  quantity  of  vapor  in  the 
atmosphere  shall  continually  vary.  To  appreciate  its  changes  we  must 
examine  the  process  of  evaporation  ;  the  distribution  of  the  vapor  through  the 
atmosphere ;  and  the  processes  by  which  it  may  be  condensed  again  into  the 
liquid  or  solid  state. 

The  process  of  evaporation  or  the  change  from  the  solid  or  liquid  to  the 
gaseous  state  requires  the  expenditure  of  a  large  amount  of  energy.  Lique- 
faction or  the  change  from  the  solid  to  the  liquid  state  also  requires  a 
considerable  supply  of  energy,  but  with  this  process  we  are  not  so  much 
concerned.  The  melting  of  ice  and  snow  must  be  duly  considered,  but  the 
evaporation  of  water  is  of  greater  importance  in  meteorology. 

It  is  supposed  that  the  energy  needed  in  evaporation  of  water  is  expended 
in  overcoming  the  attraction  that  exists  between  the  molecules  while  the  water 
is  in  the  liquid  state.  The  supply  of  energy  to  do  this  hidden  work  often 
comes  from  the  sensible  heat  of  some  adjacent  substance.  When  water 
evaporates  from  the  sea  or  from  a  lake  or  river  or  from  the  wet  surface  of  the 
land,  the  energy  needed  to  change  its  state  may  be  derived  in  part  from  the 
heat  of  the  adjacent  water  or  land  ;  but  in  the  usual  case  of  evaporation 
proceeding  under  sunshine,  it  is  supposed  that  the  energy  of  insolation  may 
pass  directly  to  the  work  of  overcoming  the  intermolecular  attractions  of  the 
water  and  thus  changing  it  to  the  gaseous  state,  without  taking  the  intermediate 
form  of  heat.  This  is  illustrated  in  the  ordinary  experience  of  a  drying  day 
after  a  rainstorm.  The  surface  of  the  land,  everywhere  wet  from  the  rain  that 
fell  from  the  clouds  of  the  day  before,  is  then  shone  upon  by  the  sun's  direct 
and  indirect  rays  from  the  clear  sky.  Instead,  however,  of  there  being  a  rapid 
rise  of  temperature,  there  is  a  rapid  drying  of  the  ground;  the  energy  of 
insolation  received  upon  the  surface  of  the  ground  is  expended  in  changing  tin- 
state  of  the  water  more  than  in  increasing  the  molecular  activity  of  the 
ground  or  of  the  water.  In  the  same  way,  the  strong  insolation  absorbed  at 
the  surface  of  the  torrid  oceans  is  devoted  more  to  causing  evaporation  than  to 
raising  the  temperature  of  the  water  :  hence  in  good  part  for  this  reason  the 
oceans  around  the  equator  are  relatively  cool. 


THE    MOISTURE    OF    THE    ATMOSPHERE.  141 

Water  vapor  is  lighter  than  the  other  gases  of  the  atmosphere.  The  weight 
of  a  cubic  foot  of  vapor  at  a  given  temperature  and  under  a  given  pressure  is 
only  0.630  of  the  weight  of  the  same  volume  of  air  under  the  same  conditions. 
A  cubic  foot  of  moist  air  is  therefore  lighter  than  a  cubic  foot  of  dry  air  at  the 
same  temperature  and  pressure  ;  but  when  water  evaporates  into  a  given 
volume  of  air,  the  weight  of  the  mixture  and  its  expansive  force  are  both 
increased  by  the  presence  of  the  vapor.  As  vapor  is  formed  from  a  wet 
surface,  it  spontaneously  but  rather  slowly  mixes  with  the  air  ;  this  process 
being  called  diffusion.  The  distribution  of  vapor  through  the  atmosphere  is, 
however,  chiefly  controlled  by  the  movement  of  the  air  in  its  local  and  general 
circulations. 

167,  Latent  heat.  The  energy  required  to  change  a  substance  from  the 
solid  to  the  liquid  state,  or  from  the  liquid  to  the  gaseous  state,  is  called  Latent 
heat.  This  term  is  essentially  a  misnomer.  It  was  introduced  into  the  science 
of  physics  at  a  time  when  very  different  ideas  concerning  the  nature  of  heat 
prevailed  from  those  that  are  now  current.  It  was  then  thought  that  heat 
was  an  imponderable  form  of  matter,  and  that  this  imponderable  matter  had  to 
enter  any  liquid,  as  water  for  example,  in  the  process  of  evaporation  ;  but  as 
evaporation  is  not  attended  necessarily  by  a  rise  of  temperature,  the  heat  thus 
entering  the  water  was  said  to  become  latent.  As  now  understood,  heat  is  not 
a  form  of  matter,  but  a  condition  of  matter,  and  latent  heat  is  not  heat,  but  is 
simply  the  energy  needed  to  overcome  the  intermolecular  attractions  of  the 
evaporating  substance ;  yet  the  name,  latent  heat,  is  still  universally  employed. 
The  student  must  be  careful  to  avoid  being  misled  by  its  apparent  meaning. 
It  is  simply  a  special  name  for  the  energy,  from  whatever  source,  expended 
in  a  certain  task  ;  its  addition  does  not  cause  any  change  of  temperature 
whatever. 

The  amount  of  energy  required  to  melt  a  pound  of  ice  at  32°  would  raise  a 
pound  of  water  from  32°  to  172°.  If  a  pound  of  fresh  snow  at  a  temperature 
of  32°  were  placed  in  a  pound  of  water  at  172°,  the  resulting  two  pounds  of 
water  would  have  a  temperature  of  32°.  Hence  the  latent  heat  of  liquefaction 
of  a  pound  of  ice  is  equal  to  140  units  of  heat,  —  a  unit  of  heat  being  the 
amount  needed  to  raise  the  temperature  of  a  pound  of  water  one  degree  Fahr. 
The  importance  of  this  in  explaining  the  low  polar  temperatures  in  summer 
under  a  strong  supply  of  insolation  has  already  been  referred  to  in  Section  91. 

The  amount  of  heat  required  to  evaporate  a  pound  of  water  is  much 
greater  than  that  required  to  melt  a  pound  of  ice.  About  a  thousand  units  of 
heat  are  needed  to  transform  a  pound  of  water  into  a  pound  of  vapor ;  the 
precise  amount  varying  with  the  temperature  at  which  evaporation  takes 
place.  At  32°  it  is  1092  units  ;  at  212°  it  is  966.  The  latent  heat  of  water 
vapor  is  therefore  very  high. 


142  ELEMENTARY  METEOROLOGY. 

With  this  in  mind,  we  may  recall  the  small  diurnal  change  of  temperature 
in  the  surface  waters  of  the  ocean.  Even  under  the  strong  shining  of  an 
equatorial  sun,  the  surface  of  the  sea  in  the  torrid  zone  warms  but  two  or 
three  degrees  from  night  to  day,  while  land  surfaces  in  the  same  latitude  may 
warm  by  twenty  or  thirty  degrees.  The  share  of  insolation  that  is  absorbed 
at  the  ocean  surface  goes  for  the  most  part,  not  to  exciting  the  molecules  of 
the  water  to  faster  motion,  that  is,  to  heating  the  water,  but  to  the  other  task 
of  changing  the  state  of  the  water  from  liquid  to  gas,  that  is,  to  supplying 
the  latent  heat  needed  for  evaporation  from  the  water  surface. 

168.  Capacity.     The  quantity  of  vapor  that  can  exist  in  the  air  depends 
upon  the  pressure  that  is  exerted  upon  it  and  upon  its  temperature ;  but  it  is 
important  to  notice  that  the  controlling  pressure  is  only  that  which  is  exerted 
by  the  vapor  itself,  and  not  by  other  gases  with  which  it  may  be  mixed. 
Thus  at  a  certain  temperature,  as  80°,  such  as  occurs  frequently  in  the  lower 
air  over  the   torrid   oceans,   the  vapor   may  increase   in   quantity  until  its 
expansive  force  equals  about  one  inch  of  barometric  pressure.     If  the  total 
pressure  is  then  thirty  inches,  the  expansive  force  of  the  lower  air  with  whicli 
the  vapor  is  mixed  will  be  twenty-nine  inches.     Hence,  although  the  tempera- 
ture of  the  air  determines  the  temperature  of  the  vapor  and  thus  controls  its 
amount,  the  pressure  of  the  air  has  no  effect  in  determining  the  quantity  of 
vapor  that  may  be  formed,  although  it  is  important  in  diminishing  the  rate  of 
evaporation,  because  it  retards  the  rate  of  diffusion  through  the  air.     It  is  as 
if  the  molecules  of  the  other  gases  of  the  atmosphere  acted  only  as  so  many 
obstacles  which  the  molecules  escaping  from  the  water  surface  had  to  pass  by ; 
for  the  total  quantity  of  vapor  that  could  be  formed  at  a  temperature  of  80° 
would  be  just  as  great  if  the  air  were  absent  as  in  its  presence.     Inasmuch, 
however,  as  the  air,  into  which  evaporation  proceeds,  in  nearly  all  cases  deter- 
mines the  temperature  of  the  vapor,  it  is  natural  and  usual  to  speak  of  the 
capacity  of  the  air  for  vapor.     This  is  really  a  misleading  expression,  like 
many  others  inherited  by  science  to-day  from  earlier  years,  but  its  convenience 
warrants  its  adoption. 

169.  Saturation.     When  the  full  capacity  of  a  given  volume  of  air  for 
vapor  has  been  reached,  the  air  is  said  to  be  saturated  with  vapor.     A  more 
careful  statement  of  this  condition  would  be  given  in  the  phrase,  the  vapor  is 
saturated;  for  it  is  believed  that,  if  any  additional  evaporation  should  take 
place  under  such  conditions,  some  of  the  preexistent  vapor  must  return  to  the 
liquid  state.     This  phrase,  however,  is  seldom  employed  ;    it  is  sufficient  to 
say  that  the  air  is  saturated.     This  state  is  therefore  spoken  of  as  saturation. 

The  capacity  of  air  for  vapor  increases  rapidly  with  rise  of  temperature. 
At  a  temperature  of  zero,  Fahrenheit,  the   expansive   force  of  the  greatest 


THE    MOISTURE    OF    THE    ATMOSPHERE. 


143 


quantity  of  vapor  that  can  then  exist  can  be  no  more  than  0.04  inch ;  at 
freezing  it  is  0.19  inch;  at  90°,  1.41.  The  following  tables  exhibit  this 
relation  more  fully,  and  add  also  the  maximum  weight  of  vapor  in  grains  per 
cubic  foot,  and  in  grams  per  cubic  meter,  at  various  temperatures  ;  and  the 
weight  of  a  cubic  foot  of  saturated  air  under  a  pressure  of  30  inches. 

PRESSURE  AND  WEIGHT  OF  VAPOR  AND  SATURATED  AIR. 


TEMPERATURE. 

VAPOR  PRESSURE. 

VAPOR  WEIGHT, 

SAT.  AIR  WEIGHT, 

Cu.  FT. 

Cu.  FT. 

°F. 

Inches. 

Grains. 

Grains. 

-30° 

0.010 

0.12 

650 

—  20 

.017 

0.21 

634 

—  10 

.028 

0.35 

620 

0 

.045 

0.54 

606 

+  10 

.071 

0.84 

593 

20 

.110 

1.30 

580 

30 

.166 

1.97 

568 

40 

.246 

2.86 

556 

50 

.360 

4.09 

544 

60 

.517 

5.76 

533 

70 

0.732 

7.99 

521 

80 

1.022 

10.95 

509 

90 

1.408 

14.81 

497 

+  100 

1.916 

19.79 

487 

TEMPERATURE. 

VAPOR  PRESSURE. 

VAPOR  WEIGHT, 
Cu.  MET. 

SAT.  AIR  WEIGHT, 
Cu.  MET. 

•C. 

mm 

Grams. 

Kilogr. 

-30° 

0.38 

0.44 

1.45 

-20 

0.94 

1.04 

1.40 

-10 

2.15 

2.28 

1.35 

0 

4.57 

4.87 

1.30 

+  10 

9.14 

9.36 

1.25 

20 

17.36 

17.15 

1.20 

30 

31.51 

30.08 

1.15 

+  40 

54.87 

50.67 

1.11 

A  room  measuring  twenty  feet  square  by  ten  feet  high  would  contain 
4,000  cubic  feet  of  air.  If  it  were  saturated  with  vapor  at  a  temperature 
of  60°,  the  weight  of  the  moist  air  would  be  304  pounds  avoirdupois ; 
and  the  vapor  would  weigh  3.3  pounds.  If  all  the  vapor  were  condensed, 
it  would  produce  91.4  cubic  inches  of  water,  or  somewhat  more  than  three 
pints. 


144  ELEMENTARY    METEOROLOGY, 

It  is  important  to  notice  that  the  increase  of  capacity  is  much  faster  at 
high  temperatures  than  at  low  temperatures.  In  a  rise  of  temperature  from 
— 10°  to  0°,  the  increase  is  only  0.19  grain  per  cubic  foot ;  from  30°  to  40°. 
the  increase  is  0.89  grain  per  cubic  foot ;  from  80°  to  90°,  the  increase  is  3.8(> 
grains,  or  more  than  twenty  times  as  much  as  in  the  first  and  four  times  as 
much  as  in  the  second  example.  At  ordinary  temperatures,  the  capacity 
doubles  for  a  rise  of  about  18°  in  temperature. 

170.  Humidity.  The  state  of  the  air  with  respect  to  the  vapor  that  it 
contains  is  called  its  humidity  :  the  humidity  is  said  to  be  high  when  the  air 
is  damp,  and  low  when  the  air  is  dry.  It  is  commonly  the  case  that  the  air 
over  the  land  does  not  possess  as  much  vapor  as  it  might,  but  on  cool,  damp 
nights  the  air  near  the  ground  is  often  saturated.  This  is  not  because  more 
vapor  is  present  then  than  in  the  day-time,  but  because  of  the  fall  of  tempera- 
ture from  day  to  night,  by  which  the  capacity  of  the  air  is  reduced  so  far  that 
the  amount  of  vapor  already  present  saturates  the  air.  On  the  ocean,  particu- 
larly in  the  calms  of  the  doldrums,  the  air  is  nearly  saturated  all  the  year  round. 
This  is  because  the  inflowing  trades,  blowing  over  the  warm  surface  of  the 
ocean  and  warming  slowly  as  they  advance  from  either  side,  are  continually 
supplied  with  vapor,  so  as  to  maintain  them  in  an  almost  saturated  condition. 
While  loitering  in  the  doldrums,  their  vapor  is  even  more  increased.  Even 
the  slight  cooling  of  the  air  over  the  equatorial  ocean  at  night  is  therefore 
sufficient  to  make  it  excessively  damp.  On  the  other  hand,  in  desert  regions 
the  supply  of  moisture  is  so  small  that  the  quantity  of  vapor  present  is  far 
from  satisfying  the  capacity  of  the  air.  There  is  very  little  present  compared 
to  that  which  might  exist ;  the  air  is  then  relatively  dry.  The  same  condition 
occurs  frequently  in  our  cold  northwest  winds  of  winter.  These  come  from  a, 
northern  interior  region,  where  their  temperature  is  very  low,  and  where  but 
little  moisture  is  present.  They  then  advance  rapidly  into  milder  latitudes, 
warming  as  they  come,  but  coming  so  quickly  that  their  increasing  capacity 
for  vapor  is  not  satisfied.  They  frequently  do  not  contain  half  as  much  vapor 
as  they  might,  and  sometimes  this  fraction  falls  as  low  as  a  third.  The  upper 
regions  of  the  atmosphere  are  also  found  to  be  prevailingly  dry.  This  serins 
to  be  the  case  even  over  the  ocean,  and  must  be  regarded  as  the  result  of  th« 
remoteness  of  the  great  volume  of  the  atmosphere  from  the  ocean  surface,  and 
of  the  obstruction  that  the  air  presents  to  the  upward  diffusion  of  vapor. 

The  humidity  of  the  atmosphere  exercises  a  strong  control  over  our  bodily 
sensation  of  the  temperature  of  the  air.  The  body  does  not  act  lik'1  a 
thermometer,  readily  accepting  the  temperature  of  the  surrounding  medium, 
but  attempts  to  maintain  an  internal  temperature  of  about  !)<S°,  known  an 
"blood  heat,"  at  all  seasons.  We  prevent  an  uncomfortable  reduction  of 
temperature  in  cold  air  by  sheltering  the  body  from  loss  of  heat  by  a  covering 


THE   MOISTURE    OF    THE   ATMOSPHERE.  145 

of  clothing ;  if  the  air  is  windy,  more  protection  is  needed  than  when  it  is 
calm  ;  if  it  is  damp  as  well  as  cold  and  windy,  it  abstracts  all  the  more  heat 
from  us,  probably  by  means  of  the  better  conductivity  given  both  to  the  air 
and  to  the  clothing  by  the  moisture ;  hence  the  difference  between  the  bracing 
though  severe  cold  of  our  dry  northwest  winter  winds,  and  the  penetrating, 
searching  chill  of  our  damp  winter  northeasters.  The  difference  between  the 
so-called  "  dry  cold  "  of  the  interior  and  the  "  damp  cold  "  of  the  New  England 
coast  is  thus  explained.  On  the  other  hand,  when  the  air  is  warm,  our  bodily 
temperature  would  rise  too  high  if  it  were  no4"  for  the  cooling  of  the  skin  by 
continual  evaporation  from  its  surface.  In  very  hot  and  very  dry  air,  the 
evaporation  is  so  much  hastened  that  the  skin  is  parched  and  burned ;  in  hot 
aiivl  very  damp  air,  evaporation  is  checked  and  the  air  feels  sultry  and  oppres- 
sive. Moderately  dry  hot  air  is  less  uncomfortable  than  at  either  of  the 
extremes  of  dryness  or  dampness.  The  oppressiveness  of  our  "dog-day" 
weather  in  July  and  August  depends  as  much  on  its  humidity  as  on  its  heat. 

The  action  of  water  vapor  on  insolation  and  terrestrial  radiation  has  been, 
much  discussed.  It  may  be  regarded  as  diathermanous  to  insolation,  but  rela- 
tively opaque  to  terrestrial  radiation,  and  it  is  therefore  thought  to  exert  a 
controlling  influence  in  determining  the  temperature  of  the  atmosphere.  It 
is  for  this  reason,  as  well  as  for  the  reasons  stated  on  pages  29  and  33,  that 
the  range  of  temperature  is  large  in  arid  regions,  and  small  over  the  oceans. 
Without  water  vapor,  the  temperature  of  the  earth  would  probably  be  much 
lower  than  that  now  prevailing.  This  appears  to  be  confirmed  by  observations 
on  the  diurnal  range  of  temperature  under  varying  conditions  of  humidity.  If  the 
temperature  of  the  air  is  well  above  saturation,  the  range  is  relatively  strong  ; 
if  near  saturation,  the  range  is  diminished  even  though  no  visible  clouding  of 
the  sky  occurs  ;  if  a  thin,  hazy  cloud  is  formed,  the  range  is  greatly  reduced. 

171.  Absolute  and  relative  humidity.  In  order  to  measure  the  relative 
dryness  or  dampness  of  the  air,  it  is  customary  to  determine  the  ratio  of 
the  amount  of  vapor  actually  present  to  that  which  might  be  present  at  the 
existing  temperature.  The  amount  of  vapor  actually  present  is  called  the 
absolute  1m.  nullify.  This  may  be  expressed  either  in  the  expansive  force  that 
the  vapor  exerts  or  in  its  weight  in  grains  per  cubic  foot  of  .air.  The  absolute 
humidity  divided  by  the  amount  of  vapor  that  might  exist  if  the  air  Were 
saturated  gives  a  ratio  that  is  called  the  relative  humidity.  Close  over  the 
ocean  surface,  the  relative  humidity  is  generally  over  90  per  cent,  and  may 
reach,  at  night,  100  per  cent ;  that  is,  the  condition  of  saturation.  In  our  dry 
winter  weather,  the  relative  humidity  may  fall  below  50  per  cent,  and  some- 
times as  low  as  40  or  even  30  per  cent.  In  deserts,  20  per  cent  is  not 
uncommon ;  and  at  noon-time,  when  the  rapid  rise  of  temperature  gives  the 
air  a  greatly  increased  capacity  for  vapor  which  cannot  be  satisfied,  the  relative 


146  ELEMENTARY   METEOROLOGY. 

humidity  may  fall  to  10  per  cent ;  indeed,  cases  are  reported  where  it  is  even 
less  than  three  per  cent ;  but  in  such  remarkable  examples  it  is  possible  that 
the  means  of  determining  the  percentage  of  humidity  have  been  at  fault. 

An  amount  of  vapor  sufficient  to  cause  a  high  relative  humidity  at  a  low 
temperature  would  cause  only  a  low  relative  humidity  at  a  high  temperature. 
An  amount  of  vapor  that  is  sufficient  to  produce  only  about  25  per  cent 
relative  humidity  at  a  temperature  of  80°  will  suffice  to  saturate  the  air  when 
its  temperature  falls  to  40°.  With  a  given  absolute  humidity,  the  relative 
humidity  will  fall  as  the  temperature  rises,  and  vice  versa :  as  the  air  warms 
during  the  morning,  the  relative  humidity  falls,  but  towards  sunset  when  the 
air  cools,  the  relative  humidity  rises  again. 

172.  Dew-point.     When  the  air  is  cooling  from  noon-day  heat  to  evening 
cold,   its   capacity  for  vapor  is  continually  decreasing.      At  noon-time,  the 
amount  of   vapor   present   seldom   satisfies   the   capacity,   and  the  air  then 
commonly  has  with  us  a  relative  humidity  of  fifty  to  eighty  per  cent;  but 
with  the  afternoon  fall  of  temperature  and  decrease  of  capacity,  the  vapor 
present  approaches  and  finally  reaches  the  stage  of  saturation  ;  the  temperature 
is  then  said  to  have  fallen  to  the  dew-point.     Any  further  cooling  will  cause 
condensation  and  produce  cloud,  fog,  dew  or  frost.     The  difference  between 
the  temperature  of  the  air  and  the  dew-point  is  called  the  complement  of  the 
dew-point.      To   this    subject  we    shall  return  after  a  consideration  of  the 
measurement  and  distribution  of  the  humidity  of  the  atmosphere. 

173.  Amount  of  evaporation.     This  section  of  our  subject  is  sometimes 
called  atmidometry.     The  amount  of  water  passing  into  the  atmosphere  by 
evaporation  varies  greatly  in  different  places,  according  to  the  temperature 
and  humidity  of  the  air,  the  strength  of  the  wind,  and  the  character  of  the 
water  surface.     Evaporation  from  surfaces  whose  temperature  is  the  same  as 
that  of  the  air  is  retarded  in  the  presence  of  damp  air;   but  as  free  water 
surfaces  are  at  night  generally  warmer  than  the  air  thai  is  resting  on  them, 
their  evaporation  continues  at  about  the  same  rate  as  dm-ing  the  warmer  hours 
of  the  day,  in  spite  of  the  dampness  of  the  night  air.     Evaporation  is  slow  in 
the  quiet  shaded  air  of  a  forest ;  it  is  small  but  does  not  cease  in  the  cold  dry 
air  of  clear  winter  weather  ;  even  snow  and  ice  may  yield  vapor  to  the  air  in 
cold  weather  without  melting. 

Evaporation  may  proceed  almost  continuously  from  a  free  water  surface, 
except  at  times  of  rain.  From  ordinary  land  surfaces,  the  vapor  passes  off 
rapidly  after  a  rain,  but  its  production  soon  decreases  as  the  surface  dries; 
further  supply  then  depends  on  the  capillary  ascent  of  water  from  beneath  the 
surface;  this  practically  ceases  during  a  drought  when  the  ground  is  dried  to  a 
considerable  depth  and  its  surface  is  parched  and  hard.  Evaporation  from  the 


THE    MOISTURE   OF    THE    ATMOSPHERE.  147 

ground  is  increased  by  plowing  or  otherwise  breaking  up  its  surface.  Plants 
exude  water  from  their  leaves  in  the  growing  season ;  deep-rooted  trees  and 
grasses  bring  a  large  amount  of  ground  water  up  to  the  air  in  this  way. 
Evaporation  is  active  from  high  mountain  surfaces  in  clear  weather  in  spite 
of  their  low  temperature,  because  of  the  open  exposure  of  their  surface  to  the 
dry  and  active  currents  of  the  upper  atmosphere. 

The  amount  of  evaporation  that  may  take  place  from  a  free  water  surface 
continually  exposed  to  the  air  has  been  determined  for  various  parts  of  the 
world.  The  measures  are  not  closely  comparable,  because  they  come  in  some 
cases  from  the  loss  by  evaporation  from  large  reservoirs,  in  which  the 
temperature  of  the  water  may  differ  from  that  of  the  air  and  thus  exercise 
a  large  control  on  its  evaporation ;  and  in  other  cases  they  are  determined  by 
the  loss  from  comparatively  a  small  volume  of  water  in  a  shallow  vessel, 
whose  temperature  may  follow  that  of  the  air  within  a  few  degrees.  The 
former  method  is  more  useful  in  connection  with  engineering  works,  such  as 
reservoirs  for  supplying  cities,  or  for  storing  water  to  be  used  in  irrigation. 
At  inland  stations  of  dry  regions  the  amount  thus  determined  does  not 
correspond  to  any  natural  quantity,  but  in  the  neighborhood  of  the  sea  or  of 
large  lakes  it  serves  to  measure  roughly  the  amount  of  water  that  passes 
from  their  surface  into  the  atmosphere.  At  stations  near  the  continental 
coasts  the  amount  of  evaporation  is  generally  a  little  less  than  the  annual 
rainfall ;  but  there  must  be  many  local  exceptions  to  this  rule.  The  records 
for  a  number  of  stations  are  here  given  ;  those  for  our  western  interior  basins 
are  general  averages. 

ANNUAL  EVAPORATION  FROM  FREE  WATER  SURFACES  IN  INCHES. 

PLACE.  LATITUDE.        EVAPORATION. 

Madras 13  N.  91.2 

St.  Helena 17    S.  83.8 

Dijon,  France 47  N.  26.2 

London 51  N.  20.6 

Boston 42  N.  39.1 

Lake  Michigan 44  N.  22.- 

Great  Salt  Lake 41  N.  80.- 

Interior  Basin,  U.  S 36  N.  150.- 

Interior  Basin,  U.  S 44  K  60.- 

Ft.  Conger 82  N.  8.9 

174.  Hygrometry.  Hygrometers  are  instruments  for  measuring  the 
yi  humidity  of  the  air.  The  simplest  instruments  of  this  kind  employ  some 
'^hygroscopic  substance,  —  that  is,  one  which  easily  absorbs  moisture  from 
•'damp  air,  —  such  as  a  hair  from  which  the  natural  oil  has  been  extracted  by 
)  placing  it  in  an  alkaline  solution.  The  hair  is  fastened  at  one  end,  passed  over 
(an  easily  rotating  cylinder,  and  held  tense  by  a  weight  or  spring  at  the  other 


148  ELEMENTARY    METEOROLOGY. 

end.  The  cylinder  carries  an  index  at  one  end  which  moves  over  a  dial, 
When  the  air  is  damp,  the  hair  absorbs  moisture  and  increases  in  length  :  the 
cylinder  is  thus  turned  and  the  index  moves  upon  the  dial.  When  the  air 
becomes  dryer,  moisture  passes  from  the  hair  and  it  shortens,  when  the 
cylinder  and  the  index  turn  the  other  way.  This  instrument  is  easily 
constructed  and  serves  to  give  a  general  indication  of  the  moisture  of 
the  atmosphere.  It  is  not,  however,  employed  in  accurate  measurements, 
except  at  temperatures  near  freezing,  when  its  records  are  to  be  preferred  to 
those  of  the  psychrometer,  described  in  the  next  section. 

A  very  simple  means  of  determining  the  dew-point  in  air  at  ordinary 
summer  temperatures  and  thus,  by  means  of  physical  tables,  determining  the 
absolute  and  relative  humidity  of  the  atmosphere,  is  as  follows  :  A  shining 
tin  cup  about  half  full  of  water  at  the  temperature  of  the  air  receives 
additional  water  poured  in  slowly  from  an  ice-pitcher.  A  thermometer  tube, 
used  as  a  stirring  rod,  mixes  the  cool  with  the  warm  water  and  continually 
indicates  the  temperature  of  the  mixture.  As  the  temperature  of  the  mixture 
is  decreased,  a  sudden  clouding  of  the  bright  surface  of  the  cup  will  be 
noticed,  and  if  the  thermometer  is  then  read  it  indicates  closely  the  tempera- 
ture of  saturation,  or  the  dew-point  of  the  surrounding  air.  It  means  that 
the  cup  and  the  air  next  to  it  have  been  cooled  a  little  below  the  temperature 
at  which  the  existing  vapor  saturates  the  air.  By  means  of  the  table  in  the 
next  section,  the  weight  of  saturated  vapor  in  grains  per  cubic  foot  may  l>e 
determined  for  the  temperature  of  the  dew-point.  The  weight  of  vapor  per 
cubic  foot  at  the  temperature  of  the  air  will  be  somewhat  less,  and  must  l»e 
calculated  from  the  known  rate  of  expansion  of  vapor  with  increase  of 
temperature.  This  calculated  quantity  divided  by  the  weight  of  a  cubic  foot 
of  saturated  vapor  at  the  temperature  of  the  air,  also  taken  from  the  table 
on  page  150,  gives  the  relative  humidity  for  the  time  of  observation.  This 
method  serves  nicely  to  illustrate  the  meaning  of  dew-point  and  saturation, 
and  may  often  be  employed  in  the  summer  time ;  but  quite  another  method  is 
adopted  for  routine  observations. 

175.  The  psychrometer.  This  instrument  is  in  general  use  for  the  deter- 
mination of  humidity  ;  observations  being  taken  at  the  usual  hours  for  records 
of  temperature.  It  depends  upon  the  facts  that  as  saturation  is  approached, 
evaporation  goes  on  more  and  more  slowly,  and  that  rapid  evaporation  calls 
for  much  more  heat  from  the  adjacent  surface  than  slow  evaporation.  Two 
thermometers  are  exposed  to  the  air.  arranged  as  in  Fig.  4(1;  one  is  the 
on li nary  thermometer  used  for  determining  the  temperature  of  the  air;  the 
bulb  of  the  other  thermometer  is  covered  with  a  thin  linen  l>a^.  kept  moist 
by  a  wick  which  connects  it  with  a  vessel  of  water  ne;n-  by.  \Vhen  the  air 
is  saturated  with  moisture,  no  evaporation  will  take  place  from  the  wet 


THE    MOISTURE    OF    THE    ATMOSPHERE. 


149 


bulb  thermometer,  and  hence  it  will  read  the    same  number  of    degrees    as 

its  neighbor  ;  but  in  dry  air,  evaporation  will  take  place  so   rapidly  from  the 

moist  surface  of  the  bulb  as  to  maintain  its  tem- 
perature several  degrees  lower  than  that  indicated 

for  the  air  by  the  other  thermometer.     The  differ- 
ence between  the  dry  and  the  wet  bulb  may  then 

be  used  with   tables  constructed  from  laboratory 

experiments   to  determine  several    quantities  de- 
sired ;  the  dew-point,  the  relative  and  the  absolute 

humidity.      The    chief    difficulty    in    using    the 

psychrometer  consists  in  determining  the  proper 

reading  of  the  wet  bulb  thermometer,  on  account 

of  the  uneven  movement  of  the  surrounding  air. 

Under   given   conditions  of   humidity,  if    the  air 

is  stagnant,  evaporation  from  the  wet  bulb  may 

cause  an  approach  to  saturation  in  the  air  close 

about  it.     Further  evaporation   is   thus  retarded, 

and    the   depression  of  the  wet  bulb   reading   is 

reduced.    On  the  other  hand,  when  the  atmosphere 

is  of  the  same  humidity  as  before,  but  is  in  active 

motion,  fresh  bodies  of  air  are  continually  carried 

past   the    wet   bulb ;    evaporation    is    much   more 

active,  and  the  depression  of  the  wet  bulb  reading 

is  decidedly  increased.  It  is  for  this  reason  that  some 
standard  rate  of  air-movement  past  the  psychrometer  should 
be  maintained.  This  is  sometimes  accomplished  by  creating 
an  artificial  draft  by  means  of  a  fan  driven  at  a  constant 
speed  by  clockwork.  A  simpler  and  much  cheaper  method 
consists  in  attaching  the  psychrometer  to  a  string  a  few  feet 
long  (Fig.  47),  and  whirling  it,  at  a  velocity  of  twelve  or 
fifteen  feet  a  second,  around  the  hand.  This  must  be  con- 
tinued in  the  open  air  until  a  constant  difference  is  main- 
tained between  the  readings  of  the  two  thermometers. 
Results  obtained  in  this  way  are  regarded  as  trustworthy, 
although  even  at  best  the  measures  of  humidity  are  of 
relatively  local  value.  At  temperatures  near  the  freezing 
point,  the  indications  of  the  hair  hygrometer  are  thought  to 
be  more  accurate  than  those  of  the  psychrometer. 

The  following  table,  taken  from  a  much  more  extended 
one  by  Hazen,  is  adapted  to  the  readings  of  a  whirled 
psychrometer.  It  gives  the  dew-point  and  the  relative 
humidity  for  different  air  temperatures  and  for  various 


FIG.  46. 


FIG.  47. 


150 


ELEMENTARY   METEOROLOGY. 


differences  between  the  whirled  dry  and  wet  thermometers ;  it  may  be  used 
as  giving  a  general  indication  of  the  values  of  these  quantities  at  altitudes 
less  than  3,000  feet.  More  extended  tables  should  be  employed  in  reducing 
regular  observations. 


DIFFERENCE  OF  READINGS 
OF  DRY  AND  WET  BULBS. 

TEMPERATURE  OF  AIR  —  FAHRENHEIT. 

-10° 

0° 

10° 

20° 

30° 

40° 

50° 

60° 

70° 

80° 

90° 

100° 

,  (  D.P.   .   .   . 

—as 

—  7 

5 

16 

27 

38 

48 

58 

69 

79 

89 

91> 

MR.H.  .  .  . 

55 

71 

80 

86 

90 

92 

93 

94 

95 

96 

96 

97 

0  (  D.P.   .   .   . 

-76 

-18 

-  1 

12 

24 

35 

46 

57 

67 

77 

87 

98 

2|R.H.  .   .   . 

10 

42 

60 

72 

79 

84 

87 

89 

90 

92 

92 

93 

C  D.P. 

_ 

-39 

—9 

7 

21 

33 

44 

55 

66 

76 

86 

96 

i  R.H.   .   .   . 

- 

13 

41 

58 

68 

76 

80 

84 

86 

87 

88 

90 

I  D-P. 

_ 

_ 

-22 

1 

17 

30 

42 

53 

64 

74 

85 

95 

{  R.H.   .   .   . 

- 

- 

21 

44 

58 

68 

74 

78 

81 

83 

85 

86 

fi(D.P.   .   .   . 

_ 

_ 

_ 

-18 

7 

24 

37 

49 

61 

72 

82 

93 

6iR.H. 

_ 

__ 

16 

38 

52 

61 

68 

72 

75 

78 

80 

_ 

-8 

16 

31 

46 

57 

68 

79 

90 

1  R  H 

18 

37 

49 

58 

64 

68 

71 

74 

10(D.P.   .   .   . 

I 

~_ 

I 

4 

25 

40 

53 

65 

77 

87 

IU}R.H.  .  .  . 

- 

- 

- 

- 

- 

22 

37 

48 

55 

61 

65 

68 

,Q  j   D.P.      .      .      . 

_ 

_ 

_ 

_ 

_ 

-16 

17 

35 

49 

62 

74 

85 

(  R.H.   .   .   . 

- 

- 

- 

- 

- 

8 

26 

39 

48 

54 

59 

62 

(  D.P. 

_ 

_ 

_ 

_ 

_ 

_ 

5 

28 

45 

58 

70 

82 

14  (  R.H.   .   .   . 

- 

- 

- 

- 

- 

- 

16 

30 

40 

47 

53 

57 

,g  (  D.P.   .   .   . 

_ 

_ 

_ 

_ 

_ 

-B 

-20 

20 

39 

54 

67 

79 

|  R.H.   .   .   . 

- 

- 

- 

- 

- 

- 

5 

21 

33 

41 

47 

51 

«g  (  D.  P.   .   .   . 

_ 

_ 

_ 

_ 

_ 

_ 

_ 

8 

33 

50 

63 

76 

(R.H.   .   .   , 

- 

- 

- 

- 

- 

- 

- 

13 

26 

36 

41 

47 

ft  (  D.  P. 

_ 

_ 

_ 

_ 

_ 

_ 

_ 

-13 

25 

45 

60 

73 

(  R.H.   .   .   . 

- 

_ 

- 

_ 

- 

- 

- 

5 

19 

29 

36 

42 

(  D  F 

_ 

_ 

__ 

_ 

_ 

_ 

_ 

16 

39 

56 

69 

22  |  R.H.   .   .   . 

- 

- 

- 

- 

- 

- 

- 

- 

12 

23 

32 

37 

I  D>p 

_ 

•  _ 

_ 

_ 

_ 

_ 

_ 

_ 

0 

32 

61 

66 

'"•"•  '  '  ' 

- 

- 

- 

- 

— 

— 

— 

— 

6 

18 

26 

33 

176.  Distribution  of  vapor  in  the  atmosphere.  Vapor  is  distributed 
through  the  atmosphere  by  two  processes.  One  of  these  is  the  spontaneous 
diffusion  of  vapor,  even  when  the  air  is  calm.  It  is  probably  in  great  part  by 
this  process,  aided  by  the  increased  expansive  force  of  the  newly  added  vapor, 
that  the  water  evaporated  from  the  torrid  oceans  in  the  calm  morning  air  of 
the  doldrums  is  prompted  to  ascend  to  higher  levels  and  form  clouds  in  the 
afternoon.  Diffusion,  however,  is  a  slow  process  compared  to  the  distribution 
of  vappr  by  the  movement  and  intermingling  of  tin-  ;iir:  and  as  the 
circulation  of  the  atmosphere  is  continually  maintained  in  a  thorough 


THE   MOISTURE   OF    THE   ATMOSPHERE.  151 

the  vapor  that  is  formed  in  one  part  of  the  world  is  gradually  carried  hundreds 
of  miles  away.  Were  it  not  for  the  various  processes  by  which  the  vapor  is 
condensed  to  the  liquid  or  solid  state  again,  the  atmosphere  would  have  long 
ago  become  completely  saturated.  But  the  limitation  of  evaporation  chiefly 
to  the  under  surface  of  the  atmosphere  where  it  rests  on  the  oceans,  coupled 
with  the  various  means  of  condensation,  generally  prevents  the  occurrence  of 
saturation  through  the  great  body  of  the  atmosphere  ;  and  in  continental 
interiors,  far  from  the  great  oceans,  as  well  as  in  the  lofty  atmosphere  high 
above  sea-level,  the  air  is  prevailingly  dry. 

177,   Geographic   and  periodic  variations  of   absolute  humidity.      The 

quantity  of  vapor  in  the  lower  air  in  the  doldrums  over  the  oceans  is  on  the 
average  greater  than  elsewhere  in  the  world,  because  of  the  intensity  of 
insolation,  the  presence  of  a  water  surface,  and  the  quietness  of  the  air.  It  is 
constantly  near  the  value  of  saturation.  In  the  trade  wind  belts  on  either 
side  of  the  doldrums,  the  vapor  is  of  somewhat  less  quantity  ;  not  so  much  on 
account  of  their  lower  temperature,  as  from  their  continual  motion,  whereby 
the  moist  lower  currents  are  mixed  with  the  next  overlying  and  drier  currents. 
In  the  horse  latitudes,  still  lower  measures  of  humidity  are  found,  partly  by 
reason  of  the  lower  temperature,  partly  because  of  the  gentle  downward  settling 
of  the  air  in  these  belts  from  high  levels  where  the  humidity  is  small.  They 
are  thus  in  strong  contrast  to  the  doldrums.  In  passing  poleward  through  the 
prevalent  westerly  winds,  the  absolute  humidity  falls  as  the  temperature 
decreases  and  in  the  polar  regions  its  amount  is  very  small  compared  with 
that  of  the  torrid  zone. 

On  the  lands,  the  absolute  humidity  varies  first  with  the  prevailing 
temperatures,  second  with  the  distance  from  the  oceans  from  which  the 
prevailing  winds  blow,  third  with  the  degree  of  enclosure  by  mountains,  and 
fourth  with  altitude  above  sea-level.  The  forested  lowlands  of  the  Amazon 
possess  damp  winds  flowing  in  from  the  moist  regions  of  the  torrid  Atlantic. 
The  northwest  coasts  of  Europe  and  North  America  have  nearly  as  high  an 
amount  of  vapor  as  their  temperatures  will  allow  in  winter,  when  their  winds 
come  from  the  warmer  currents  of  the  oceans  next  westward.  The  interior  of 
Europe  and  the  lowlands  of  western  Asia  have  progressively  less  and  lees 
humidity,  because  the  vapor  brought  from  the  Atlantic  has  been  in  part 
abstracted  on  the  way  by  condensation,  and  the  rest  has  been  mixed  with  dryer 
upper  winds.  Our  far  northwestern  plains,  whose  winds  come  chiefly 
from  the  west,  have  low  humidity,  not  only  because  they  are  far  inland  from 
the  Pacific,  but  also  because  of  the  mountain  barriers  to  windward;  for 
mountains  are  very  effective  in  abstracting  vapor  from  the  air.  The  greater 
part  of  our  interior  basin  is  a  desert,  in  spite  of  its  moderate  distance  from 
the  ocean,  because  of  the  great  height  to  which  the  Sierra  Nevada  rises 


152  ELEMENTARY   METEOROLOGY. 

between  it  and  fhe  Pacific.  The  interior  depression  of  Asia,  enclosed  on  all 
sides  by  lofty  mountains,  is  an  arid  waste  because  so  great  a  share  of  the  vapor 
in  the  winds  that  approach  it  has  been  left  on  the  outer  slopes  of  the  ranges. 

The  quantity  of  vapor  in  the  upper  air  is  small  ;  it  rapidly  diminishes  as 
one  ascends  to  greater  and  greater  altitudes.  Even  in  regions  where  the  lower 
air  is  well  supplied  with  vapor,  the  upper  strata  possess  very  little.  This  is 
partly  due  to  the  low  temperature  at  great  heights  ;  but  it  depends  also  on  the 
small  ratio  of  the  evaporating  surface  of  the  ocean  to  the  volume  of  the  air, 
and  on  the  slowness  with  which  vapor  spreads  by  diffusion  through  the  air. 
When  the  pressure  of  vapor  at  sea  level  is  one  inch,  or  one  thirtieth  of  the 
total  pressure  of  the  atmosphere,  it  cannot  be  inferred  that  the  vapor  present 
at  high  levels  exerts  the  same  share  of  the  pressure  there  experienced.  If  a 
perfect  calm  prevailed,  such  a  condition  might  be  approached,  but  numerous 
observations  in  the  moving  atmosphere  on  mountains  and  in  balloons  have 
shown  that  the  decrease  of  vapor  pressure  upwards  is  much  faster  than  the 
value  of  vapor  pressure  at  sea  level  would  indicate.  The  density  of  the  vapor 
in  the  lower  air  is  maintained,  not  only  by  the  pressure  of  the  vapor  above 
it,  but  also  by  the  resistance  that  the  atmosphere  opposes  to  the  free  upward 
diffusion  of  vapor  from  the  lower  strata. 

The  amount  of  vapor  in  the  air  is  generally  greater  in  the  warmer  season 
than  in  the  colder.  This  is  particularly  the  case  in  those  regions  where  the 
warm  season  has  inflowing  winds  from  the  sea,  as  in  India.  There  the  summer 
monsoon  is  a  warm,  moist,  vapor-bearing  wind,  while  the  winter  monsoon  is 
cooler  and  comparatively  free  from  vapor. 

The  average  diurnal  variations  of  the  amount  of  vapor  are  small.  The 
amount  is  somewhat  greater  by  day  than  by  night  in  regions  where  evaporation 
is  hastened  under  sunshine  ;  but  such  changes  are  greatly  exceeded  by  the 
accompanying  unperiodic  shifts  of  the  winds,  by  which  the  air  is  brought  over 
the  observer  from  a  new  source.  Thus  our  southerly  warm  winds  bring  a  large 
amount  of  vapor  from  the  Gulf  of  Mexico  and  from  the  warm  ocean  waters 
near  our  southern  Atlantic  states ;  while  our  cold  northwesterly  winds  bring 
little  vapor  from  the  continental  interior,  as  will  be  further  explained  in 
the  chapter  on  Weather. 

178-  Geographic  and  periodic  variations  of  relative  humidity.  The 
variations  of  relative  humidity  are  often  unlike  those  of  absolute  humidity. 
The  warm  doldrums  have  a  high  relative  (83  per  cent  or  more)  and  a  hijjli 
absolute  humidity;  hence  the  air  is  sultry  and  oppressive.  The  trade  winds 
over  the  ocean  have  a  lower  relative  humidity  (77  per  cent)  than  the  doldrums  : 
on  land,  if  they  blow  over  a  rising  surface,  they  are  moist;  but  if  they  pass 
over  lowlands,  they  are  dry  and  the  region  is  reduced  to  a  desert.  The  quiet 
air  of  the  horse  latitudes  is  somewhat  drier  than  that  of  the  trades.  The 


THE    MOISTURE   OF    THE   ATMOSPHERE.  153 

westerly  winds,  with  much  lower  absolute  humidity  than  the  trade  winds,  have 
a  high  relative  humidity  over  the  oceans  (90  per  cent  or  more),  and  in  their 
stormy  areas  they  are  saturated  with  vapor.  A  low  relative  as  well  as  a  low 
absolute  humidity  has  often  been  recorded  in  the  north  polar  regions.  The 
upper  air  is  relatively  dry,  as  has  been  explained  above. 

The  periodic  variations  of  relative  humidity  are  as  a  rule  the  reverse  of 
those  of  absolute  humidity.  In  the  warmer  season,  the  capacity  for  vapor 
generally  increases  in  a  higher  degree  than  the  amount  of  vapor.  Thus 
continental  interiors  are  as  a  rule  drier  in  summer  and  by  day  than  in  winter 
and  by  night.  If  observations  are  taken  in  rapid  succession  through  a  day, 
the  considerable  variations  that  may  be  detected  are  often  to  be  ascribed  to 
convectional  currents,  by  which  masses  of  air  from  upper  and  lower  levels  are 
alternately  carried  past  the  observer. 

179.   Effect  of  water  vapor  on  the  general  circulation  of  the  atmosphere. 

The  prevailing  excess  of  water  vapor  in  the  lower  equatorial  atmosphere  has  a 
small  effect  in  aiding  the  circulation  of  the  general  winds.  Recalling  the 
greater  elasticity  of  vapor  than  of  air,  it  follows  that  where  vapor  is  in  excess 
the  isobaric  surfaces  will  be  held  a  little  further  apart  than  where  it  is 
deficient ;  recalling  further  that  the  absolute  humidity  over  the  great  oceanic 
surface  of  the  world  increases  rapidly  with  the  temperature,  it  follows  that 
the  divergence  of  the  isobaric  surfaces  already  explained  as  a  consequence  of 
high  equatorial  temperatures  (Section  111)  will  be  a  little  further  increased  in 
consequence  of  the  high  equatorial  value  of  the  absolute  humidity  ;  and  that 
a  similar  assistance  will  be  given  to  the  continental  winds  of  winter  when  the 
air  over  the  oceans  is  moister  as  well  as  warmer  than  over  the  lands.  This 
effect  is,  however,  insignificant  when  compared  with  that  of  the  equatorial  and 
polar  or  the  oceanic  and  continental  contrasts  of  temperature.  It  may  be 
confidently  asserted  that  if  the  earth  had  no  oceans,  and  hence  no  currents  by 
which  the  contrast  between  torrid  and  frigid  temperatures  is  reduced,  the 
poleward  temperature  gradients  would  be  much  stronger  than  we  find  them ; 
and  the  gain  in  the  velocity  of  the  terrestrial  circulation  thus  produced 
would  much  more  than  compensate  for  the  loss  following  the  withdrawal  of 
the  aid  now  given  by  the  mere  presence  of  water  vapor. 

It  is  important  to  notice  that  as  far  as  water  vapor  acts  on  the  circulation 
of  the  winds,  the  effect  varies  with  the  absolute  and  not  with  the  relative 
humidity  of  the  air.  In  spite  of  the  high  relative  humidity  of  the  damp  and 
chilly  atmosphere  in  high  southern  latitudes  and  the  apparent  great  amount 
of  vapor  there  present,  the  actual  amount  of  vapor  is  much  less  than  in  the 
clear  air  of  the  warm  trade  winds.  It  is  therefore  inadmissible  to  ascribe  the 
low  Antarctic  pressures  to  the  presence  of  water  vapor,  as  some  have  done. 


154  ELEMENTARY  METEOROLOGY. 

CHAPTER   IX. 

DEW,    FROST    AND    CLOUDS. 

180,  Condensation.     The  natural  processes  of  condensation  of  the  water 
vapor   in  the  atmosphere  all  depend  on  a  decrease  of  temperature.     It  is 
possible  in  the  laboratory  to  produce  condensation  by  the  compression  of 
moist  air,  the  temperature  in  the  meantime  being  maintained  at  a  constant 
value ;  but  this   process  has  practically  no  application  in  meteorology ;  for 
when  air   containing  vapor  is  compressed,  as   in  a  descending  current,  the 
diminution  of  capacity  resulting  from  decrease  of  volume  is  more  than  made 
up  by  the  increase  of  capacity  resulting  from  increase  of  temperature  ;  and 
descending  currents  for  this  reason  soon  become  dry. 

Condensation  as  a  result  of  cooling  the  air  has  already  been  briefly  referred 
to  in  Section  172.  Any  mass  of  air  containing  vapor  will,  if  cooled  sufficiently, 
be  reduced  to  the  dew-point,  and  if  the  cooling  then  proceeds  further,  progres- 
sive condensation  will  accompany  it.  If  condensation  occurs  at  'temperatures 
above  32°,  the  product  will  be  water ;  if  below  32°,  it  will  be  ice  in  the  form 
of  frost,  snow  or  hail. 

181.  Condensation  from  quiet  air  on  cold  surfaces.     There  are  various 
processes  by  which  the  temperature  of  the  air  is  cooled  to  the  point  of  con- 
densation.    One  of  the  simplest  of  these  is  illustrated  in  the  cooling  that 
accompanies  the  diurnal  changes  of  temperature  in  the  atmosphere.     These 
have  been  already  explained  to  be  small  in  the  upper  air  and  greatest  near  the 
ground,  and  the  latter  occurrence  will  therefore  be  first  considered.     As  the 
temperature  rises  in  the  morning,  any  moist  surface  rapidly  yields  its  vapor 
to  the  warming  air.     As  a  rule,  however,  the  increase  of  temperature  goes  on 
so  rapidly  that  the  supply  of  vapor  does  not  cause  saturation.     During  the 
morning  hours,  even  though   the  absolute  humidity  increases  slightly,   the 
relative  humidity  quickly  falls.     During  the  first  hours  of  the  afternoon,  if 
evaporation  continue  from  the  warmed  ground  into  the  warm  air,  the  absolute 
humidity  still  increases  slowly,  but  at  this  time  the  temperature  is  falling  and, 
the  capacity  of  the  atmosphere  for  vapor  is  thereby  decreasing;   hence  tin 
relative  humidity  rapidly  increases.     About  sunset  in  well-watered  regions 
the  air  close  to  the  ground  is  nearly  saturated,  as  we  may  know  from  the 
growing  dampness  of  the  grass  ;  and  from  this  time  on  the  further  cooling  of 
the  ground  during  the  night  and  the  consequent  cooling  of  the  air  next  to  it 
by  radiation  and  conduction,  causes  the  continuous  deposition  of  vapor  in  tho 
form  of  dew  or  frost ;  the  former  if  condensation  occurs  at  temperatures  above 


DEW,    FROST    AND    CLOUDS.  155 

32°,  the  latter  at  lower  temperatures.  As  vapor  is  thus  withdrawn  at  the 
bottom  of  the  atmosphere,  an  additional  supply  is  furnished  by  downward 
diffusion  from  above,  and  condensation  is  maintained  continuously  on  the 
cooling  surface.  In  this  it  is  like  the  formation  of  water  drops  upon  the  cold 
surface  of  an  ice-pitcher  in  a  warm  room :  the  warm  air  of  the  room  corresponds 
to  the  great  body  of  little-cooled  air  above  the  earth's  surface ;  the  cold 
surface  of  the  ice-pitcher  corresponds  to  the  cooling  surface  of  the  ground  at 
night ;  and  the  drops  of  water  upon  the  pitcher  represent  the  part  of  the  dew 
that  is  condensed  from  the  air  upon  the  ground. 

182.  Cooling  retarded  by  the  liberation  of  latent  heat.     Whenever  water 
vapor  is  condensed,  there  is  as  much  energy  set  free  as  was  expended  in  the 
production  of  the  vapor.     This  is  called  the  liberation  of  latent  heat.     The 
greater  intensity  of  a  scald  from  steam  at  212°  than  from  water  at  212°  is  thus 
explained.     It  has  been  stated  in  Section  167  that  the  latent  heat  required  for 
evaporation  may  be   supplied  directly  by  the  energy  of  insolation  without 
taking  the  intermediate  form  of  heat.     It  is  equally  true  that  the  latent  heat 
liberated  in  condensation  may  at  once  take  the  form  of  radiant  energy  and 
not  appear  as  heat  at  all.     In  the  example  of  the  preceding  section,  the 
liberated  latent  heat  does  not  raise  the  temperature  of  the  body  on  which 
condensation  takes  place ;    it  merely  supplies  a  certain  share  of  the  radiant 
energy  that  is  emitted  from  the  surface  of  the  body,  and  thus  diminishes  the 
rate  at  which  radiation  is  supplied  from  the  heat  of  the  body  itself.     Hence 
the  decrease  of  temperature  both  of  the  surface  on  which  the  air  rests  and  of 
the  lower  air  also  is  slower  after  condensation  begins  than  before ;  and  it  is 
partly  for  this  reason  that  moist  regions  have  a  small  diurnal  range  of  tem- 
perature ;  their  greater  cloudiness  or  haziness,  and  the  retardation  of  warming 
by  evaporation  in  the  day-time  and  of  cooling  by  condensation  at  night  all 
conspire  to  this  end.     Arid  regions,  on  the  other  hand,  have  extreme  diurnal 
ranges   Of  temperature.      Southeastern  California  and  the  adjacent  part  of 
Arizona  have  an  average  summer  diurnal  temperature  range  of  40°  or  45° ;  the 
greatest  known  in  the  world.     The  retardation  of  cooling  by  the  latent  heat 
of  condensation  is  an  extremely  important  matter  in  meteorology  and  will  be 
frequently  encountered  in  later  pages. 

183.  Dew.     The  formation  of  dew  has  given  rise  to  much  discussion,  but 
since  the  early  years  of  this  century,  the  experiments  of  Wells  have  generally 
been  accepted  as  giving  a  satisfactory  explanation  of  its  origin,  substantially 
as  stated  in  the  preceding  paragraphs.     Recent  experiments  by  Aitken  and 
others  show,  however,  that  only  part  of  the  dew  and  frost  formed  on  the 
ground  comes  from  the  air ;  the  rest  comes  from  the  ground  or  from  plants. 
During  the  day-time,  under  sunshine  and  in  the  presence  of  an  active  wind, 


156  ELKMKN TAUY    METEOROLOGY. 

the  surface  of  the  ground  is  dried ;  water  rises  from  the  subsoil  by  capillarity 
to  supply  more  vapor  to  the  thirsty  air.  The  sap  passing  from  the  leaves  of 
plants  is  also  easily  disposed  of  then,  because  it  is  exuded  in  the  form  ot 
extremely  minute  drops,  and  because  it  is  generally  well  exposed  for  evapora- 
tion to  the  surrounding  air.  But  at  night,  drops  of  exuded  water  may  collect 
on  the  leaves  of  grass  and  low  plants,  where  it  is  unable  to  evaporate  in  the 
cold  nocturnal  air ;  and  water  rising  to  the  surface  of  the  bare  ground  may 
remain  there,  instead  of  passing  off  as  vapor.  If  the  ground  is  covered  with 
grass,  the  blades  of  grass  become  colder  than  the  ground  beneath  them ;  the 
vapor  rising  from  the  ground  will  then  be  in  good  part  condensed  on  the  cold 
grass. 

There  is  a  large  variation  in  the  proportion  of  dew  supplied  from  its 
several  sources  at  different  times  and  places.  In  damp  countries,  the  share 
coming  from  the  ground  is  large  ;  in  dry  regions,  where  the  dew  formed  at 
night  may  be  a  large  part  of  the  water  on  which  the  growth  of  plants  depends, 
the  greater  part  of  it  presumably  comes  by  condensation  from  the  air.  Few 
careful  observations  have  yet  been  made  on  this  subject. 

184.  Frost  is  formed  in  the  same  manner  as  dew,  but  at  temperatures 
below  the  freezing  point.  The  frost  that  forms  just  below  the  surface  of  the 
ground,  raising  the  surface  soil  by  the  growth  of  its  crystals,  is  probably 
derived  for  the  most  part  from  the  subsoil :  that  which  is  deposited  on  loose 
leaves  and  sticks  lying  on  the  ground  may  also  be  supplied  in  good  part  from 
the  ground ;  the  share  that  then  comes  from  the  air  is  not  yet  determined. 

Little  attention  has  been  given  to  the  measurement  of  the  diurnal  or 
annual  amount  of  dew  and  frost,  on  account  of  the  difficulty  of  the  problem. 
It  has  been  estimated  that  the  dew  of  a  single  night  equals  0.02  inch  of  rainfall 
(Sect.  283)  ;  and  that  the  total  annual  amount  of  dew  precipitated  in  Great 
Britain  would  measure  an  inch  and  a  half  in  depth. .  Further  study  should  be 
given  to  this  subject. 

185.  Conditions  for  the  formation  of  dew  or  frost.  In  dry  regions,  even 
when  the  range  of  temperature  is  great  and  the  nights  are  much  cooler  than 
the  days,  it  may  be  that  the  dew-point  is  not  reached  at  night  and  no  dew  is 
formed.  This  is  commonly  the  case  in  deserts  ;  but  in  regions  of  moderate 
aridity,  where  the  rainfall  is  of  small  quantity  and  the  days  are  warm  and 
dry,  the  nights  may  often  be  cool  enough  to  form  dew  or  frost,  as  on  <>ui 
western  plains,  and  on  the  pampas  of  the  Argentine  Republic.  In  well-watered 
regions,  such  as  the  eastern  United  States,  the  formation  of  a  moderate  amount 
of  dew  or  frost  in  favorable  situations  is  common  on  all  clear  and  quiet  nights, 
and  it  is  accompanied  by  inversions  ot  temperature  of  greater  or  less  strength. 
The  damp  air  of  the  torrid  zone  yields  abundant  dew  on  many  equatorial  lands 


DEW,    FROST    AND    CLOUDS.  157 

in  the  so-called  "  dry  season,"  when  the  sky  is  prevailingly  clear.    The  weather 
exerts  a  strong  control  on  the  formation  of  dew.     The  cooling  of  the  ground 
and  of  the  lower  air  will  be  greatest  on  clear  nights,  when  there  is  no  haze  or 
cloud  to  dimmish  the  effective  radiation  to  outer  space.     The  cooling  will  be 
greater  on  calm  than  on  windy  nights,  for  when  the  lower  stratum  of  air  lies 
quiet  it  is  cooled  more  and  more  as  the  night  goes  on  ;  while  when  the  wind 
blows,  a  stratum  that  has  been  somewhat  cooled  by  contact  with  the  cooling 
ground  is  carried  away,  and  replaced  by  another  stratum  from  above  that  is 
less  cooled.     The  special  case  of  mountain  frost-work  in  windy  weather  is 
described  in  Section  193.     The  nocturnal  cooling  of  the  ground  has  already 
been  described  as  greatest  upon  convex  surfaces,  from  which  radiation  goes  on 
in  all  directions  ;  and  less  from  concave  surfaces  or  valleys,  above  which  there 
is  a  less  surface  of  open  sky.     We  might  at  first  infer  from  this  that  hills 
would  receive  more  dew  than  valleys  ;  but  it  should  be  recalled  that,  in  conse- 
quence of  the  surface  cooling,  a  descending  drainage  is  established  from  the 
hillsides  to  the  valleys,  so  that  the  cold  air  of  the  hilltops  is  drained  away  and 
continually  replaced  by  uncooled  air.     Hilltops  therefore  receive  comparatively 
little  dew.     On  the  other  hand,  the  air  that  creeps  slowly  down  the  slopes 
into  the  valleys  is  continually  cooled  by  conduction  to  the  ground  as  it  moves 
on.     It  is  true  that  at  the  same  time  an  increase  of  pressure  upon  it  tends  to 
raise  its  temperature  by  compression  ;  but  as  the  descent  of  the  air  is  generally 
rather  slow,  this  cause  of  increase  of  temperature  is  less  effective  than  the 
cooling  by  conduction  to  the  cold  ground  during  the  gradual  descent.     Yet 
again,  if  the  downward  nocturnal  drainage  be  concentrated  in  a  narrow  valley, 
leading  from  a  large  upland  surface  to  an  open  lowland,  the  active  mountain 
breeze  blowing  out  of  the  valley  may  descend  so  rapidly  as  to  reach  the 
lowland  less  cooled  than  the  air  which  crept  more  slowly  down  the  adjacent 
mountain  slopes ;  in  such  a  case,  the  lowland  opposite  the  mouth  of  the  valley 
might  be  freer  from  frosts  than  the  general   lowland  surface  thereabouts. 
These  varied  examples  afford  good  illustrations  of  the  many  processes  at  work 
to  determine  the  formation  of  so  simple  a  matter  as  dew. 

186,  Prediction  of  frost.  The  occurrence  of  frost  in  the  late  spring  or 
early  fall  is  injurious  to  many  crops,  and  it  is  often  highly  important  that 
warning  should  be  given  in  the  afternoon  of  the  probable  formation  of  frost 
at  night.  The  general  method  of  foretelling  by  means  of  weather-maps  the 
occurrence  of  clear  and  quiet  nights,  when  frost  is  likely  to  be  formed,  will  be 
explained  in  Chapter  XIII ;  but  mention  may  be  made  here  of  a  simple  method 
of  prediction  applicable  by  any  .farmer.  If  the  afternoon  shows  a  decreasing 
cloudiness  and  a  weakening  wind,  the  temperature  of  the  dew-point  gives  a 
ready  means  of  inferring  the  minimum  temperature  of  the  night :  for  when  the 
dew-point  is  reached,  further  nocturnal  cooling  is  retarded,  and  therefore  if 


158  ELEMENTARY   METEOROLOGY. 

condensation  begins  at  temperatures  above  40°,  it  is  seldom  that  the  minimum 
temperature  of  the  night  will  fall  to  freezing  ;  but  no  definite  rule  can  be  given 
in  this  case.  Local  experience  is  needed  to  determine  the  "  safety  limit "  of 
the  dew-point  and  its  relation  to  the  season  and  the  weather.  Special  study 
might  be  profitably  turned  in  this  direction. 

187.  Protection  from  frost.     When  the  occurrence  of  frost  appears  likely, 
it  is  often  possible  to  protect  plants  from  injury  by  building  a  smoky  fire  on 
the  windward  side  of  a  field,  so  that  a  dense  stratum  of  smoke  may  drift 
slowly  over. the  surface.     Radiation  is  then  transferred  in  great  part  from  the 
ground   and   the   plants   to   the   smoky   stratum,   and   the   injurious  fall  of 
temperature  at  the  level  of  the  ground  is  effectively  retarded.     This  method 
is  often  practised  to  protect  tobacco  in  meadows  and  cranberries  in  low  boggy 
ground,   where  the  air  is  more  quiet  than  on  hills  or  slopes.      On  higher 
ground  such  protection  is  less  often  needed ;  for  the  more  active  movement 
of  the  air  over  hills,  coupled  with  the  conditions  of  nocturnal  inversions  of 
temperature  and  the  nocturnal  drainage  of  the  air  on  sloping  ground,  generally 
serves  to  maintain  the  minimum  temperature  in  quiet  weather  on  hillslopes 
several  degrees  above  that  experienced  in  the  neighboring  valleys  and  lowlands. 
Peach  orchards  in  the  more  northern  states  should  be  for  this  reason  generally 
planted   on   rising   ground :    many  examples    could   be   given   in  which   the 
blossoming  in  an  orchard  on  higher  ground  escaped  freezing,  while  another 
near  by  on  lower  ground  had  all  its  trees  blighted.     The  limitation  of  low 
temperatures  to  the  lower  layers  of  the  air  is  sometimes  so  marked  that  the 
upper  branches  of  small  trees,  or  shrubs  escape  a  frost  that  injures  the  lower 
branches. 

188.  Valley  and   lowland   fogs.     Valley   bottoms   are,   in   spite   of  the 
diminished  nocturnal  cooling  of  their  soil,  as  a  rule  much  damper  at  night 
than  the  adjacent  hilltops;  evaporation  may  continue  from  the  little-cooled 
surface  of  their  streams  and  ponds;  they  may  not  only  receive  a  plentiful 
deposit  of  dew  on  the  ground,  but  their  lowest  part  may  be  covered  with  a 
layer  of  mist  or  fog.     Common  experience  gives  many  examples  of  this  ;  when 
driving  late  at  night  over  a  hilly  road,  the  change  of  both  temperature  and 
dampness   from    hill  to  hollow  is    perfectly  apparent.     In  the   torrid   zone, 
where  the  diurnal  changes  of  temperature  are  peculiarly  regular,  the  occurrence 
of  nocturnal  valley  fogs  is  a  characteristic   feature  of  the  climate.     In  the 
winter  of  temperate  latitudes,  when  the  long  nights  are  interrupted  only  by 
short  days  of  weak  sunshine,  the  valleys  of  mountainous  regions  may  be  filled 
to  a  considerable  depth  with  a  cold  damp  fog ;  in  spells  of  qutet  and  cold 
winter  weather,  broad  lowlands  are  sometimes  covered  by  fog  sheets  of  this 
kind,  extending  over  thousands  of  square  miles,  attaining  a  thickness  of  a 


DEW,    FROST    AND    CLOUDS.  159 

thousand  or  more  feet,  and  remaining  for  even  a  week  at  a  time ;  producing 
extremely  raw  and  disagreeable  weather  beneath,  while  the  sun  shines  above 
through  air  of  extraordinary  clearness  (Sect.  249). 

It  sometimes  happens  that  fog  prevails  even  though  the  air  is  at  a 
temperature  several  degrees  above  its  dew-point.  This  is  known  as  a  "  dry 
fog."  It  has  been  plausibly  suggested  that  the  fog  particles  of  such  a  time  may 
have  an  oily  coating,  derived  from  the  combustion  of  coal  and  wood  in  cities, 
by  which  evaporation  is  retarded  ;  but  it  is  not  yet  proved  that  dry  fogs  occur 
only  under  conditions  where  the  products  of  combustion  are  relatively  plentiful. 
Like  the  occurrence  of  supersaturated  cloudless  air  and  of  clouds  formed  of 
water  particles,  although  the  air  in  which  they  float  may  be  several  degrees 
below  the  freezing-point,  as  has  sometimes  been  reported  by  balloonists,  dry 
fog  still  awaits  a  full  explanation. 

All  forms  of  fog  may  be  classed  between  the  dew  and  frost  that  are 
condensed  at  the  bottom  of  the  atmosphere  and  the  overhanging  clouds,  to 
the  consideration  of  which  we  now  proceed. 

189.  Dependence  of  cloud  condensation  on  "dust."     Recent  experiments 
by  Aitken  have  given  good  reason  to  think  that  the  formation  of  clouds  in 
the  open  air  is  greatly  aided  by  the  presence  of  fine  particles  of  solid  or  liquid 
matter,  or,  as  it  is  commonly  expressed,  by  the  presence  of  "dust  particles." 
If  all  suspended  particles  are  removed  from  a  volume  of  air,  its  temperature 
may  be  reduced  several  degrees  below  the  dew-point   before   condensation 
begins,  and  the  air  is  then  said  to  be  supersaturated.     This  is  not  of  common 
occurrence  in  natural  processes.     In  the  lower  air  suspended  particles  are  well 
known  to  exist  in  countless  numbers.     In  the  upper  air  at  great  heights,  the 
particles  that  may  be  present  are  not  properly  named  by  the  word  dust ;  yet 
the  best  explanation  of  the  blue  color  of  the  sky  (Sect.  65)  depends  upon  the 
presence  there,  as  well  as  in  the  lower  air,  of  matter  not  in  the  gaseous  state, 
but  in   such  excessively  fine  division  as  to  remain  suspended  in  the  air  for 
indefinite  periods.     If  condensation  of  vapor  into  clouds  requires  the  presence 
in  all  cases  of  solid  or  liquid  nuclei,  the  nuclei  must  be  of  extreme  fineness : 
for  if  rain-water  is  collected  in  a  clear  vessel,  no  perceptible  sediment  will 
settle  from  it,  unless  it  is  caught  over  some  dusty  or  smoky  region ;  yet  every 
rain-drop  has  been  formed  by  the  union  of  countless  minute  cloud  particles, 
every  one  of  which,  according  to  this  theory,  must  have  had  a  nucleus  when  its 
condensation  began.    While  the  laboratory  experiments  seem  to  leave  no  doubt 
upon  this  question,  the  natural  occurrence  of  cloud  and  rain  does  not  give  it 
unqualified  support :  the  hypothetical  nuclei  are  not  found  by  direct  observation. 

190,  Size  and  constitution  of  cloud  particles.     Clouds  formed  at  tempera- 
tures above  32°  consist  of  minute  spherical  drops  of  water,  ?Tfov  to  Tu^  of 


160  ELEMENTARY  METEOROLOGY. 

an  inch  or  more  in  diameter.  There  is  no  sufficient  reason  for  accepting  the 
old  belief  that  cloud  particles  are  hollow  vesicles.  Clouds  formed  at  tempera- 
tures below  32°  consist  of  minute  ice  spicules,  which  may  increase  in  size  and 
become  snow-flakes.  The  low  temperatures  at  which  such  clouds  occur 
prevail  in  the  upper  regions  of  the  atmosphere  all  over  the  world,  and  at 
lower  levels  in  the  winter  season  or  near  the  poles.  Cloud  particles  are 
ordinarily  so  minute  that  they  fall  very  slowly  through  even  so  light  a 
medium  as  air,  and  a  very  faint  ascending  current  is  sufficient  to  bear  them 
upward.  When  their  size  increases  by  continued  condensation,  they  may 
become  large  enough  to  fall,  slowly  at  first,  faster  afterwards  ;  and  thus  rain 
or  snow  is  produced.  The  association  of  rain  or  snow  with  storms  of  different 
kinds  will  be  considered  in  the  two  following  chapters,  after  which  a  later 
chapter  will  consider  the  occurrence  and  distribution  of  rainfall  over  the 
world. 

191.  Color  of  clouds.     The  numerous  particles  of  which  clouds  consist 
generally  reflect  so  large  a  share  of  the  rays  incident  on  them  that  they  are  of 
the  same  color  as  the  light  by  which  they  are  illuminated :  they  are  therefore 
white  in  sunlight  and  gray  in  shadow   during  the  day-time,  or  tinged  with 
bright  colors  at  sunset  or  sunrise.     The  brilliant  light  on  the  edge  of  a  cloud 
that  hides   the  sun  is  caused  by  the  diffraction  of   rays  on  the  marginal 
particles.     At  the  time  of  formation  or  disappearance,  light  fleecy  clouds 
often  take  a  purplish  tint  when  near  the  horizon  and  far  from  the   sun. 
Distant  massive  clouds  near  the  horizon  may  have  a  yellowish  or  even  a  ruddy 
tint  at  noon-time,  on  account  of  the  selective  scattering  of  the  blue  rays  from 
the  light  that  is  reflected  from  them. 

192,  Halos  and  coronas.     The  icy  nature  of  lofty  clouds  is  known  not 
only  from  observations  on  mountains  and  in  balloons,  but  also  by  the  action 
of  the  clouds  on  sunlight.     A  thin  veil  of  lofty  cloud  (cirro-stratus)  over  the 
sky  frequently  produces  a  ring  or  halo  of  light  around  a  less  illuminated 
circular  space  of  21°  radius,  with  the  sun  or  moon  in  the  center.     The  halo  is 
colored  when  well  defined,  and  then  has  red  on  the  inside  and  blue  on  the 
outside.     This  may  all  be  so  well  explained  by  reflection  and  refraction  of 
light  in  ice  crystals  that  there  can  be  no  doubt  of  the  icy  structure  of  sucli 
clouds.     In  the  polar  regions  minute  ice  crystals  are  often  scattered  through 
the  lower  air.     The  halos   then    formed  are  of   remarkable   brilliancy  and 
complicated  form.     Many  dense  and  massive  clouds  must  have  a  low  tempera 
ture  in  their  upper  parts,  and  there  they  also  must  be  of  ice. 

Coronas  are  concentric  colored  rings,  ordinarily  of  small  diameter,  with 
the  red  on  the  outside,  formed  around  the  sun  or  moon  in  clouds  of  moderate 
thickness.  These  rings  are  produced  by  the  diffraction  and  interference  of 


DE\V,    FIJOST    AND    CLOUDS.  161 

light  on  fine  particles  of  water  or  ice.  The  smaller  solar  coronas  are 
generally  lost  in  the  blinding  glare  of  light  around  the  sun.  Incomplete  arcs 
of  delicate  coronal  colors  are  often  seen  in  the  thinner  margins  of  heavier 
clouds  at  much  greater  angular  distances  from  the  center  of  light. 

193.  Cloudy  condensation  in  winds  over  cold  surfaces.     An  effective 
means  of  cooling  air  to  and  below  the  dew-point  is   seen  when  our  damp 
southerly  winds  of  winter  blow  over  a  snow-clad  surface.     The  snow  is  then 
rapidly  thawed  by  the  warmth  gained  from  the  air ;  at  the  same  time  the  air 
is  so  rapidly  cooled  that  it  is  not  unusual  for  it  to  become  foggy  close  to  the 
ground.     Fogs  over  the  cold  waters  of  the  Newfoundland  banks  are  formed 
in  the  same  way.     A  small  illustration  of  a  similar  process  is  sometimes  seen 
in  the  formation  of  a  cloud  banner  over  a  sharp  mountain  peak,  when  the  air 
brushing  past  the  summit  of  the  mountain  is  cooled  to  a  temperature  below 
its  dew-point.     A  standing  patch  of  cloud  is  the  result ;  its  particles  stream 
along  with  the  wind  beyond  the  peak,  but  before  moving  very  far  the  moisture 
condensed  by  the  cold  of  the  mountain  is  re-evaporated  by  mixture  with  the 
adjacent  air  at  the  end  of  the  cloud. 

The  cold  rocks  of  mountains  may  at  such  times  become  heavily  covered 
with  frost,  condensed  upon  them  from  the  air.  The  frost  grows  to  curious 
forms,  building  itself  out  to  windward.  On  Mt.  Washington  the  anemometer 
at  the  station  of  the  Signal  Service  formerly  maintained  there  was  often  so 
heavily  covered  with  frost  formed  in  this  way  as  to  be  useless  as  a  wind 
measure. 

194.  Clouds  formed  in  poleward  winds.     Much  larger  illustrations  of 
cloudy  condensation  occur  in  those  winds  which  move  from  a  lower  to  a  higher 
latitude.     Nearer  the  equator,  where  the  air  is  exposed  to  stronger  sunshine, 
its  temperature  is  maintained  at  a  higher  degree ;  but  as  it  advances  poleward 
and  the  relation  of  insolation  to  radiation  weakens,  the  temperature  progres- 
sively falls,  and  the  entire  mass  may  become  filled  with  a  thick  sheet  of 
clouds,  which  lies  like  a  cloak  over  thousands  of  miles  of  land  or  sea,  and 
shelters   the   lower   air   from   warming   by  day  and  from  cooling  at  night. 
Condensation  is  aided  when  the  poleward  wind  blows  from  an   ocean   over 
a   winter   continent ;    the   air   then   cools    by    radiation   downward    to    the 
ground  as  well  as  outward  to  space,  and  the  cloud  cloak  becomes  thicker. 
Such  clouds  are  well  known  during  southerly  winds  in  the  winter  weather  of 
the   eastern    United   States.     When   their   under   surface  is  illuminated  by 
oblique  lift'at,  as   at  sunset  or  sunrise,  one  may  see  fleecy  pendant   masses, 
whose  filaments  gradually  settle  down  and  dissolve  away  in  the  lower  warmer 
and  driei  air :  when  the  clouds  are  very  heavy,  they  may  yield  rain.     If  the 
cloud-m£  iing  winds  are  stormy,  fragmental   clouds,  or  "scud,"  are  formed 


162  ELEMENTARY  METEOROLOGY. 

beneath  the  heavy  cloud-sheet,  leaning  forward  with  the  wind  and  rapidly 
changing  their  form.  Northerly  winds,  whose  temperature  with  us  rises  as 
they  advance,  and  whose  capacity  for  vapor  therefore  increases,  are  on  the 
other  hand  characterized  by  a  clear  sky. 

195.  Condensation  by  the  mechanical  cooling  of  ascending  currents.     In 
all  cases  where  a  vertical  or  an  obliquely  ascending  movement  is  given  to  a 
current  of  air,  it  is  cooled  by  the  expenditure  of  some  of  its  energy  in  the  work 
of  expanding  against  the  surrounding  air  as  it  rises,  as  has  been  explained  in 
Section  49.     This    process,   it  must  be   remembered,  is  an   immediate   one, 
keeping  close  pace  with  the  ascent :  it  is  not  like  the  process  of  cooling  by 
conduction  or  radiation,  which   is    slow.      Mechanical  cooling  of   ascending 
currents    precisely  accompanies    expansion  as  the   air  rises  to   greater   and 
greater  altitudes.     This  is  probably  the  most  general  process  of  cloud-making 
that  occurs  in  the  atmosphere.     Attention  was  first  called  to  it  by  Espy,  a 
noted  American  meteorologist,  about  1835 ;    he  was  also  the  first  to  explain 
properly  the  prevailing  clearness  and  dryness  of  descending  currents,  as  well 
as  the  diurnal  period  of  the  wind  and  of  cloudiness,  now  to  be  considered. 

196.  Convectional  clouds  of  fair  summer  weather.     Mechanical  cooling  ot 
ascending  currents  is  finely  illustrated  in  the  formation  of  day-time  clouds  — 
known  as  cumulus  clouds  —  during  the  ordinary  fair  weather  of  summer  time. 
The  reader  will  recall  from  Section  54  the  account  of  the  local  convectional 
disturbances  of  the  lower  air  on  warm  summer  days ;  and  from  Section  159, 
the  application  of  this  process  to  explain  the  diurnal  variation  in  the  velocity 
of  the  lower  and  the  upper  winds  on  land.      We  must  now  examine  more 
carefully  the  formation  of  clouds  whenever  an  ascending  current  rises  so  high 
as  to  reduce  its  temperature  below  the  dew-point. 

Let  us  consider  the  case  of  a  summer  morning,  when  the  temperature  of 
the  lower  air  on  the  land  close  to  sea  level  may  be  70°,  and  its  dew-point  67°, 
giving  it  a  high  relative  humidity.  As  the  sun  shines  through  it  and  upon  the 
ground  below,  the  temperature  of  the  lower  air  rises  rapidly,  and  before  nine 
or  ten  o'clock  it  may  reach  75°  or  80°.  At  the  same  time,  evaporation  from 
the  dew-covered  surface  of  the  ground  slightly  increases  the  amount  of  vapor, 
but  this  increase  is  not  nearly  so  rapid  as  the  incivase  of  capacity,  and 
therefore,  although  the  absolute  humidity  rises  by  a  small  amount  as  the 
morning  advances,  the  relative  humidity  will  fall  distinctly.  When  the 
temperature  reaches  80°,  the  dew-point  may  hi-  08°;  the  relative  humidity  will 
then  be  much  lower  than  before.  At  this  time,  let  it  be  supposed  that  the 
warmed  lower  air  has  become  unstable,  the  vertical  temperature  gradient  of 
the  region  being  represented  by  the  curve  BKN,  in  Fig.  48.  The  lower  air 
consequently  ascends  through  the  overlying  cooler  air ;  as  it  rises,  it  cools  by 


DEW,    FROST    AND    CLOUDS. 


163 


expansion  at  the  relatively  rapid  rate  of  1°.6  for  every  three  hundred  feet  of 
vertical  ascent,  as  explained  in  Section  49,  and  here  illustrated  by  the  line 
///•'A'.  Section  197  will  consider  the  rate  of  cooling  when  the  dew-point 
is  passed  and  latent  heat  is  liberated  by  the  condensation  of  some  of  the 
vapor. 

When  such  a  convectional  process  is  well  established,  the  ascent  of  the 
lower  air  is  rapid  and  it  reaches  a  considerable  height.  If  the  air,  with 
temperature  and  humidity  as  above  stated,  ascends  about  half  a  mile,  it  will 
there  be  reduced  almost  to  its  dew-point ;  but  it  must  be  noticed  that  as  the 
air  ascends  and  expands,  its  dew-point  falls  slowly,  because  the  vapor  that  is 

carried  up  will  not  so  nearly  saturate  the 
increased  volume  of  the  expanded  air  as 
it  did  when  the  volume  was  smaller  under 
greater  pressure  near  the  ground.  The 
rate  at  which  the  dew-point  is  thus 
lowered  by  expansion  is  represented 
graphically  by  the  dotted  lines,  DE,  de, 
etc.,  Fig.  48  ;  its  numerical  value  being  a 
third  of  a  degree  Fahrenheit  for  300  feet 
of  ascent  (or  0.2°  C  for  100  meters).  But 
the  rate  of  cooling  by  expansion,  BJFK, 
is  more  rapid  than  the  falling  of  the 
dew-point,  DE,  and  the  former  will  soon 
overtake  the  latter  in  air  of  ordinary 
humidity:  in  the  example  here  figured, 
saturation  will  be  produced  —  not  at  a 
height  F,  where  the  temperature  of  the 
ascending  air  is  reduced  to  the  dew-point 
that  it  had  before  ascent  began  —  but  at 

a  height  E,  or  2750  feet,  and  at  a  temperature  of  65°.3 ;  this  being  nearly  3° 
lower  than  the  dew-point  of  the  air  before  its  ascent  began.  It  may  therefore 
be  said  that  the  height  in  feet  at  which  condensation  will  begin  in  convectional 
currents  equals  the  complement  of  the  dew-point  divided  by  (1.6  —  0.3)  X  300. 
As  the  dew-point  is  reached  and  passed,  saturation  is  followed  by  condensa- 
tion, and  the  clouds  of  the  morning  appear.  The  dust  that  aids  condensation 
in  the  open  air  is  always  present  in  ascending  currents,  unless  the  condition 
of  the  atmosphere  is  very  exceptional ;  and  hence  there  is  as  a  rule  no 
hesitation  in  the  beginning  of  the  process.  It  has,  however,  been  supposed 
that  if  the  ascending  air  should  be  extremely  clean,  the  greater  part  of  it 
might  be  cooled  by  expansion  to  temperatures  distinctly  below  the  dew-point, 
thus  making  it  supersaturated,  until  at  last  a  forced  condensation  takes  place 
and  produces  a  rapid  and  abundant  clouding.  But  under  ordinary  circumstances 


164  ELEMENTARY  METEOROLOGY. 

it  must  be  admitted  that  cloudiness  begins  immediately  after  the  dew-point  is 
reached.  The  clouds  are  ragged  at  first ;  as  they  grow  they  gain  a  heaped-up 
form,  and  are  therefore  called  cumulus  clouds.  They  nearly  always  lean 
forward  and  curl,  over  on  reaching  the  faster-moving  upper  currents.  As  they 
absorb  insolation  that  would  otherwise  pass  down  to  the  ground,  their  cooling 
is  retarded  and  their  bouyancy  increased  ;  this  being  an  important  aid  to  the 
ascent  of  all  cloudy  currents  in  the  day-time.  As  far  as  the  ascending  current 
rises  above  the  height  at  which  condensation  began,  the  cloud  continues  to 
grow  :  the  further  the  cooling  continues,  the  more  vapor  is  condensed,  the 
ascending  air  being  always  held  at  its  dew-point.  Attentive  observers  may 
easily  detect  the  inflow  at  the  base  of  these  clouds,  where  misty  wisps  thicken 
as  they  rise  and  coalesce  with  the  main  mass  ;  the  ascent  aloft  where  the 
convex  cloud  heads  grow  outward  with  visible  expansion  ;  and  the  melting 
away  of  the  cloud  as  its  foremost  parts  roll  over  and  slowly  descend. 

The  vertical  thickness  of  the  cloud  will  depend  on  the  dampness  of 
the  air  and  the  activity  of  its  convectional  ascent.  In  very  dry  air,  such  as 
occurs  over  deserts,  the  convection  may  be  active  in  the  lofty  ascent  of  dusty 
whirlwinds,  yet  no  clouds  appear  because  the  cooling  by  expansion  is  not 
enough  to  reduce  the  dry  surface  air  to  a  temperature  low  enough  to  cause 
condensation.  On  the  other  hand,  over  the  ocean  where  the  air  is  moist,  the 
height  of  incipient  condensation  may  be  less  than  a  thousand  feet.  The  flat 
under  surfaces  of  these  clouds,  at  the  altitude  at  which  condensation  begins. 
are  of  about  the  same  height  all  across  the  sky,  and  constitute  as  marked  a 
feature  of  fair  summer  weather  as  the  growing  convex  summits  of  the  clouds. 
The  height  to  which  the  cloud  will  grow  above  the  level  of  first  condensation 
depends  on  the  additional  height  to  which  the  ascending  current  rises,  and  this 
depends  in  turn  on  the  contrast  of  temperature  between  the  lower  air  and  the 
overlying  strata  through  which  it  rises.  If  the  lower  air  becomes  very  warm,  as 
at  noon-day  in  summer,  the  clouds  may  grow  to  a  truly  mountainous  height ; 
while  in  the  early  morning  the  convectional  ascent  is  so  moderate  as  to 
produce  only  small  patches  of  cloud  in  the  clear  sky  ;  and  in  winter,  when  the 
air  is  generally  stable,  clouds  of  this  kind  are  uncommon. 

On  almost  any  fair  summer  day  the  convectional  process  of  cloud-making 
may  be  watched  from  its  beginning  in  the  morning  till  it  ceases  about  sunset. 
Small  curling  clouds  and  fresh  breezes  indicate  the  establishment  of  active 
convection  at  an  early  hour;  the  clouds  increase  in  size  through  the  morning, 
and  if  the  ground  has  been  moistened  by  a  rain  the  night  before,  the  adjacent 
clouds  may  grow  to  so  great  a  size  as  almost  to  coalesce  and  overcast  the 
noon-time  sky :  but  the  clear  blue  of  the  upper  air  may  still  be  seen  in  the 
intervals  between  the  clouds.  With  their  growth,  the  morning  breezes  freshen 
and  become  a  brisk  wind  in  the  afternoon  ;  hence  the  common  name  of  "  wind 
clouds,"  often  given  to  these  forms.  The  strong  sunshine  passing  through 


DK\V,    FROST    AND    CLOUDS. 


165 


spaces  between  the  clouds  lights  up  the  dusty  afternoon  air  with  slanting 
beams  of  light,  apparently  converging  to  the  sun,  but  really  parallel.  If  the 
observer  ascend  a  lofty  mountain-slope  on  such  a  day,  he  may  reach  the  level 
of  condensation  at  which  the  bases  of  the  clouds  all  rest. 

With  the  shading  of  the  ground  by  the  cloudy  cover,  and  with  the  descent 
of  the  sun  in  the  afternoon,  the  cause  of  the  convectional  action  is  weakened, 
and  after  four  or  five  o'clock  it  has  generally  stopped.  As  the  ascending 
motion  on  which  the  growth  of  the  clouds  depends  is  lessened  and  as  the  weight 
of  the  water  particles  of  which  the  clouds  consist  depresses  the  ascending 
currents,  the  clouds  settle  down  and  dissolve  away.  Thus  the  sky  of  late 
afternoon  or  evening  is  left  as  clear  as  it  was  in  the  morning.  The  regularity 
with  which  this  process  is  repeated  in  fair  summer  weather,  especially  in  the 
fair  weather  of  the  torrid  zone,  leaves  no  doubt  of  its  dependence  on  diurnal 
sunshine  and  convection. 


197.   Decreased  rate  of  adiabatic  change  of  temperature  in  cloudy  air. 

We  must  return  now  to  examine  the  effect  of  the  liberation  of  latent  heat 
x  in  ascending  currents  of  air  when 

cloud-making  has  begun ;  and  of  the 
reverse  process  in  descending  cloudy 
currents. 

The  retardation  of  nocturnal 
cooling  when  latent  heat  is  liberated 
in  the  formation  of  dew  or  frost 
has  been  explained  in  Section  182. 
In  that  case  the  liberated  energy 
passed  away  as  terrestrial  radiation 
into  space,  without  doing  any  work 
on  the  earth.  In  the  problem  now 
before  us,  the  liberated  latent  heat 
of  an  ascending,  expanding  current 
is  applied  to  aiding  in  the  work  of 
pushing  away  the  surrounding  air ; 
the  heat  of  the  ascending  current 
is  therefore  drawn  upon  more  slowly 
to  do  its  share  in  this  work,  and 
hence  the  fall  of  temperature  in 
the  ascending  air  is  retarded.  This 
process  is  of  wide  application,  and 
FlG'  49<  must  be  carefully  considered. 

In  Fig.  49,  let  the  altitude  and  temperature  of  incipient  condensation  in  a 

mass  of  air  that  has  risen  from  sea-level  be  indicated  by  E.     In  this  case,  we 


166 


ELEMENTARY   METEOROLOGY. 


may  say  that  the  work  of  expansion,  represented  by  IE,  has  all  been  performed 
by  the  only  available  energy  ;  namely,  the  -heat  of  the  rising  air,  and  that 
the  air  has  for  this  reason  been  cooled  from  the  temperature  B  to  G.  If  the 
air  should  now  ascend  through  the  additional  height,  EM,  the  corresponding 
work  of  expansion  may  be  represented  by  ML.  There  are  now  two  available 
sources  of  energy  that  may  be  called  on  to  do  this  work ;  the  sensible  heat 
of  the  moist  air,  and  the  latent  heat  liberated  from  the  vapor  that  is  con- 
densed in  consequence  of  the  cooling  by  the  loss  of  the  sensible  heat.  It  is 
found  by  calculation  from  the  known  values  of  the  capacity  of  air  for  vapor 
and  of  the  latent  heat  of  vapor,  that  a  part,  MH,  of  the  total  work,  ML, 
requires  a  loss  of  sensible  heat  that  will  cause  the  condensation  of  an  amount 
of  vapor  whose  liberated  latent  heat  will  just  perform  the  remainder  of  the 
work,  HL.  Hence  when  condensation  begins,  the  sensible  cooling  of  the 
ascending  current  will  no  longer  be  at  the  rapid  rate,  EL,  but  at  the  retarded 
rate,  EHS. 

The  retarded  rate  of  adiabatic  cooling  has  been  determined  for  various 
temperatures  and  pressures,  as  given  in  the  following  table,  or  as  represented 
in  Fig.  49  by  the  broken  lines,  hs.  The  greatest  retardation  is  found  at  high 
temperatures  and  heavy  pressures  ;  that  is,  under  conditions  where  the  decrease 
of  capacity  with  decrease  of  temperature  is  most  rapid.  At  very  low  tempera- 
tures, the  retardation  is  insignificant. 


KATE  OF  COOLING  OF  CLOUDY  ASCENDING  CURRENTS. 
A.    DECREASE  OF  TEMPERATURE  IN  FAHRENHEIT  DEGREES  FOR  300  FEET  OF  ASCENT. 


PRESSURE. 

10° 

20° 

30° 

40° 

50° 

60° 

70° 

80° 

90° 

30"  

1.2 

1.1 

1.0 

1.0 

0.9 

0.8 

0.7 

0.6 

0.6 

26   

1.2 

1.0 

1.0 

0.9 

0.8 

0  7 

0.7 

0  6 

0  5 

22   

1.1 

1.0 

1.0 

0.9 

0.8 

0.7 

0.6 

0.5 

18   

1.0 

1.0 

0.9 

0.8 

0.7 

0.6 

B.    DECREASE  OF  TEMPERATURE  IN  CENTIGRVIH;   I)i  <-KKES  FOR  100  METERS  ASCENT. 


PRESSURE. 

—10° 

-5° 

0° 

+  6° 

10° 

15° 

20° 

25° 

30° 

760  

0.74 

0.68 

0.64 

0.68 

0.53 

0.48 

0.43 

0.40 

0  37 

700  

.73 

.66 

.63 

.57 

.51 

.46 

42 

.38 

.36 

600  ........ 

.70 

.63 

.60 

.54 

.48 

.43 

.40 

.:;<; 

500  

.66 

.60 

.66 

.50 

.45 

.40 

87 

400  

.62 

.65 

.51 

,46 

.41 

.37 

300  

.56 

.49 

.46 

.42 

200  

.48 

.41 

.::•.» 

DEW,    FROST    AND    CLOUDS.  167 

If  a  mass  of  cloudy  air  is  descending,  the  problem  is  reversed  and  the  rate 
of  adiabatic  warming  by  compression  is  retarded.  The  heat  that  would  be 
given  entirely  to  raising  the  temperature  of  the  air  if  it  were  not  cloudy  is  in 
tli  is  case  devoted  in  part  to  supplying  latent  heat  for  evaporation  of  the  cloud 
particles.  After  all  the  cloud  is  evaporated,  the  rise  of  temperature  goes  on 
at  the  usual  rapid  rate  of  1°.6  for  every  300  feet  of  descent.  An  important 
application  of  this  principle  will  be  found  in  Section  248. 

198,  Special  adiabatic  conditions  at  the  freezing-point.     A  peculiar  con- 
dition is  found  in  a  convectional  current  whose  ascent  is  so  high  as  to  cool  it 
to  the  freezing-point,  as  often  happens  in  thunder-storms.     The  cloudy  current 
of  air  is  saturated  with  vapor  and  carries  upwards  a  vast  number  of  minute 
water  particles.     When  the  freezing  temperature  is  reached  no  further  cooling 
can  take  place  until  all  the  suspended  water  particles  are  frozen ;  and  during 
this  time,  all  the  work  of  expansion  is  done  by  the  latent  heat  liberated  in  the 
change  of  the  water  from  the  liquid  to  the  solid  state.     Still  more ;  in  conse- 
quence of  the  expansion  of  the  air  during  this  time  of  no  cooling,  its  capacity 
for  Vapor  slightly  increases,  and  some  of  the  water  particles  or  ice  crystals  will 
return  to  the  vaporous  state,  deriving  their  necessary  latent  heat  from  that 
liberated  in  the  freezing  of  some  of  their  neighbors.     Hence  for  a  brief  space, 
ascent  may  be  accomplished  without  decrease  of  temperature,  as  shown  by  the 
lines,  h'h",  s's",  Fig.  49.     No  definite  statement  can  be  made  concerning  the 
height  of  ascent  without  cooling  at  this  critical  stage  of  the  process,  because 
the  amount  of  water  present  varies  with  the  size  of  the  cloud  and  with  the 
velocity  of  the  ascending  current ;   but  in  some  cases  it  may  amount  to  as 
much  as  fifty  or  a  hundred  feet.     It  is  therefore  at  most  a  trifling  matter,  and 
only  deserves  mention  from  its  theoretical  interest. 

199.  Increased  altitude  of  convectional  ascent  in  cloudy  currents.    Kecall- 
ing  the  explanations  of  Section  197,  it  is  apparent  from  Fig.  49  that  if  no  cloud 
were  formed,  the  convectional  ascent  of  the  air  there  considered  would  cease 
at   K,  where   the   adiabatic   line,  BJL,  intersects  the  vertical   temperature 
gradient,  BKN.     But  as  cloud  is  formed  after  reaching  the  altitude  E,  the 
rate  of  cooling  is  then  changed  from  EKL  to  EHS  by  the  liberation  of  latent 
heat ;  and  hence  the  ascending  air  will  not  be  reduced  to  the  temperature  of 
the  air  through  which  it  ascends  until  a  much  greater  height  than  K  is  gained. 
The  higher  the  temperature  and  the  damper  the  air,  the  more  effective  is  the 
aid  thus  given  from  the  condensing  vapor.     The  velocity  of  ascent  is  also 
increased  after  cloud-making  begins  ;  for  on  account  of  the  retarded  cooling  of 
the  cloudy  air,  its  excess  of  temperature  over  that  of  the  surrounding  air  is 
maintained  at  a  greater  value   than   it  would  be   in   an  unclouded  current. 
Application  of  this  principle  will  be  found  abundantly  in  the  chapters  on  storms. 


168  ELEMENTARY   METEOROLOGY. 

It  must  be  borne  in  mind  that  the  simple  adiabatic  changes  here  considered 
are  never  precisely  realized.  Mixture,  conduction  and  radiation  all  tend  to 
equalize  the  temperatures  of  the  ascending  and  surrounding  air,  and  thus  to 
diminish  the  altitude  attainable;  and  the  cloud  particles  act  as  a  burden  which 
holds  down  the  ascending  current  below  the  height  it  might  gain  without 
them.  On  the  other  hand,  the  cloud  particles  absorb  by  day,  when  convection 
generally  occurs,  much  insolation  that  would  otherwise  pass  on  to  the  earth, 
and  this  aids  the  retardation  of  cooling  in  the  ascending  current ;  and  the 
momentum  of  the  ascent  tends  to  carry  the  current  above  the  normal  height 
of  equilibrium,  especially  if  the  ascent  be  rapid.  Even  if  all  these  disturbing 
influences  could  be  allowed  for,  the  value  of  the  vertical  temperature  gradient 
could  seldom  be  well  ascertained,  and  hence  the  point  of  intersection  of  the 
two  critical  lines  must  be  ill  determined.  The  curves  of  Fig.  49  and  of  other 
figures  of  the  same  kind  must  be  taken  only  to  indicate  a  rough  solution  of 
the  convectional  problem  ;  but  if  thus,  understood,  they  will  be  found  of  much 
assistance  in  gaining  a  clear  idea  of  important  meteorological  processes.  The 
problem  of  the  formation  of  clouds  may  now  be  resumed  with  a  fuller 
understanding. 

200.  Convectional  clouds  over  islands  and  mountains.  Mountainous 
islands  are  often  clothed  with  diurnal  clouds  while  the  surrounding  air  is 
comparatively  clear.  This  is  because  the  combined  action  of  the  inflowing 


FIG.  50. 

sea  breeze  and  the  ascending  valley  breeze  carries  the  damp  air  from  over  the 
ocean  up  to  a  height  at  which  it  becomes  cloudy.  A  cloud-ring  of  this  origin 
has  been  described  over  the  island  of  Hawaii  in  the  tropical  Pacific  ocean 
(Sect.  256). 

Either  of  the  two  processes  of  cloud-making  here  in  operation  may  produce 
clouds  when  acting  alone.  Arms  of  the  land,  like  Cape  Cod,  may  be  marked 
out  in  the  summer  sky  by  the  growth  of  floating  cumulus  clouds  in  quiet 
weather.  At  the  same  time,  the  sky  over  the  mainland  to  the  northwest  is 
heavily  charged  with  large  clouds,  while  over  the  sea  the  sky  is  eh-ar,  except 
that  in  quiet  weather  isolated  clouds  grow  over  the  s:mdy  island  of  Nantucket, 
Fig.  50,  the  island  itself  not  being  visible  fnmi  the  mainland.  The  sand-bars 
enclosing  Pamlico  and  Albemarle  sounds  should  determine  the  development 


DEW,    FROST   AND    CLOUDS.  169 

of  clouds  in  calm  summer  weather  in  the  same  way.  On  the  other  hand, 
inland  lakes  may  produce  clear  sky,  when  the  surrounding  land  is  cloud-covered. 
InHnd  mountain-ranges  are  generally  obscured  in  the  afternoons  by  the  forma- 
tion of  clouds  around  their  summits,  often  growing  to  the  size  of  thunder 
storms.  Mountain  climbers,  knowing  this,  make  their  ascents  as  early  as 
possible  to  gain  a  more  extended  view. 

201,  Varied  form  of  convectional  clouds.  The  rate  of  ascent  of  the 
convectional  cloud-forming  current  is  often  so  slow  and  the  burden  of  cloud 
particles  is  so  heavy  a  load  for  it  to  bear  up  that  the  summit  of  the  cloud  fails 
to  reach  the  altitude  where  its  temperature  would  be  reduced  to  that  of  the 
air  about  it,  and  where  it  might  then  spread  out  horizontally,  after  the  fashion 
of  dust  whirlwinds.  In  such  cases,  the  cloud  topples  over  and  dissolves  away. 
Where  the  activity  of  ascent  is  moderate,  and  the  horizontal  dimensions  of 
the  cloud  exceed  the  vertical,  it  is  called  strato-cumulus,  Fig.  51.  The  top  of 
a  more  active  cloud  mass  some- 
times spreads  out  laterally  at  a 
moderate  altitude,  forming  a  high, 
flat  cloud  with  definite,  sharp-cut 
margin.  It  is  probable  that  this 
form  is  best  developed  when  the 
altitude  to  which  the  cloud  cur- 
rent rises  is  determined  not  only 
by  its  own  cooling  but  also  by 
reaching  a  relatively  warm  upper 

current,  into  which  it  cannot  rise,  and  by  whose  more  rapid  forward  motion 
the  top  of  the  cumulus  cloud  is  brushed  out  into  a  horizontal  sheet.  Sheet 
clouds  of  this  origin  often  float  away  for  many  miles,  gradually  breaking  into 
alto-cumulus  masses  and  slowly  dissolving,  but  remaining  visible  long  after 
their  original  cumulus  source  has  disappeared.  The  flat  layer  clouds  of  sunset 
are  often  the  remains  of  outspreading  clouds  formed  in  this  way. 

If  the  cumulus  cloud  rises  into  damp  upper  air,  a  thin  sheet  of  cirro-stratus 
cloud  may  be  formed  over  the  rising  summit  of  the  cumulus  mass,  soon  to  be 
broken  through  as  the  cumulus  rises  higher. 

If  the  convectional  ascent  be  excessive,  as  in  thunder-storms  (Sect.  254), 
the  cloud  mass  may  attain  an  extraordinary  volume  and  altitude,  being  then 
called  cumulo-nimbus.  Some  such  clouds  have  been  charted  over  a  length  of 
two  or  three  hundred  miles,  while  their  summit  height  has  been  determined  at 
six  or  eight  miles.  A  broad  outflow  of  soft  fleecy  cirro-stratus  cloud  floats 
away  from  the  top ;  it  is  generally  unsymmetrical,  reaching  further  forward 
with  the  upper  wind  in  the  direction  of  the  advance  of  the  storm.  When 
fibrous  at  the  edges,  slowly  curling  and  eddying,  it  is  called  cirrus.  The  under 


170  ELEMENTARY   METEOROLOGY. 

surface  of  the  cirro-stratus  cloud  is  sometimes  festooned  by  the  settling  down 
of  misty  layers  from  its  under  surface  into  the  clear  air  beneath.  The  lower 
portion  of  such  a  great  thunder-storm  cloud  consists  of  water-drops  ;  but  the 
upper  portion  may  be  of  snow  even  in  summer,  as  is  proved  by  the  snow 
falling  from  thunder-storms  on  lofty  mountains,  while  the  valleys  receive  only 
rain.  Examples  of  much  larger  cloud-masses  in  connection  with  tropical 
convectional  storms  will  be  given  in  Section  218  ;  they  need  mention  in  this 
connection  because  of  the  great  development  of  long,  feathery,  cirrus  clouds 
that  radiate  from  the  lofty  storm-cloud  mass  for  many  miles  around,  forming 
a  stratiform  shield  at  heights  of  five  or  more  miles.  They  are  matted  together 
near  the  stormy  area,  and  are  there  called  cirro-stratus  ;  further  away  they  are 
more  fibrous  and  feathery,  and  are  known  as  cirrus.  Their  form  changes  very 
slowly.  They  appear  to  be  the  final  product  of  the  cooling  by  ascent,  perhaps 
aided  by  mixture  with  the  cold  lofty  air,  as  they  are  formed  where  the  central 
ascending  component  of  the  motion  gradually  turns  outward  to  a  far-reaching, 
nearly  horizontal  movement.  The  small  change  observed  in  such  clouds  from 
day  to  night  implies  that  warming  by  the  diurnal  absorption  of  insolation  and 
cooling  by  nocturnal  radiation  has  but  a  moderate  effect  in  producing  or 
transforming  them  ;  but  the  stratiform  portion  of  these  lofty  sheets  sometimes 
breaks  up  into  patches,  as  if 'prompted  to  local  convectional  movement  by 
arrested  insolation  ;  the  little  fleecy  clouds  thus  formed  being  called  cirro- 
cumulus  (see  also  Sect.  203). 

202.  Clouds  in  forced  ascending  currents.  The  ascent  by  which  clouds 
are  formed  need  not  be  of  spontaneous  convectional  origin,  as  in  the  cases 
just  considered.  Any  process  by  which  the  air  is  raised  to  levels  of  less 
pressure  will  serve,  if  it  continues  far  enough.  Thus  when  the  damp  trade 
winds  blow  against  a  mountain  slope,  and  rise  to  pass  over  it,  they  soon 
become  cloudy.  If  the  winds  are  strong  and  the  mountains  high,  the  clouds 
may  grow  so  large  as  to  give  forth  rain. 

Promontories  facing  a  windward  sea  are  for  the  same  reason  frequently 
cloudy.  Table  Mountain  at  the  Cape  of  Good  Hope  is  spread  over  by  a  sheet 
of  cloud,  known  as  the  "  table-cloth,"  when  damp  winds  blow  over  it ;  the 
cloud  grows  as  the  wind  ascends  on  the  windward  side,  and  dissolves  away  as 
it  descends  to  leeward.  Clouds  of  this  kind  sometimes  do  not  touch  the 
mountain  over  which  they  are  produced ;  and  from  this  it  may  be  inferred 
that  only  certain  ones  of  the  arching  atmospheric  strata  are  damp  enough  to 
pass  the  dew-point  as  they  rise.  It  sometimes  happens  that  a  comparatively 
dry  wind  passes  over  a  mountain  crest  and  draws  up  damp  currents  on  the 
leeward  slope,  which  then  become  cloudy. 

In  the  blustering  winds  of  March,  when  local  convection  is  becoming 
active  witli  the  rapid  warming  of  the  ground  and  the  lower  air  while  the 


DEW,    FROST    AND    CLOUDS. 


171 


upper  air  is  still  cold,  the  rolling  of  the  lower  currents  may  often  carry  a 
convectional  updraft  for  a  short  time  to  a  greater  height  than  it  could  reach 
in  a  still  atmosphere  ;  thus  forming  the  ragged  clouds  of  that  windy  season. 
Many  of  the  tangled  cirrus  clouds  (Fig.  52),  floating  at  great  heights  and 
changing  their  form  slowly,  may  perhaps  be  ascribed  to  a  driven  ascent  in 
conflicting  currents  (see  also  Sect.  208).  Indeed,  reason  will  be  given  in  a  later 
chapter  for  thinking  that  many  of  the  great  masses  of  heavy  clouds  with 
long,  outspreading  cirrus  plumes,  measuring  one  or  two  hundred  miles  in  length, 


30°  Above  Horizon— 

FIG.  52. 

that  form  over  the  stormy  areas  of  the  prevailing  westerly  winds  of  the 
temperate  zones,  particularly  in  winter  time,  may  be  in  great  part  examples 
of  condensation  in  a  driven  whirling  ascent  on  a  large  scale  (Sect.  237). 

203.  Clouds  formed  in  atmospheric  waves.  When  adjacent  air  currents 
move  with  different  velocities  or  in  different  directions,  their  surface  of 
contact  may  be  thrown  into  a  series  of  slowly  oscillating  waves  of  consider- 
able horizontal  length  from  node  to  node,  but  of  moderate  vertical  amplitude 
of  oscillation.  The  waves  of  the  sea  surface  are  produced  by  the  difference 
in  velocity  of  wind  and  water  ;  the  ripples  on  the  surface  of  sand  dunes  and 
of  snow  drifts,  as  well  as  of  sand  bars  under  water,  are  of  similar  origin ;  the 
violent  agitation  of  the  sea,  known  as  "rips,"  where  tidal  or  other  currents 
conflict,  are  of  the  same  kind.  Waves  in  the  atmosphere  are  generally  of 
very  slow  movement.  Their  existence  may  be  recognized  in  the  undulating 
form  of  the  under  surface  of  broad  cloud  sheets.  The  regular  spacing  of 
clouds  between  equal  intervals  of  clear  sky  also  depends  on  some  undula- 
tory  movement  of  atmospheric  strata.  Indeed,  atmospheric  wave  motion  is 


172  ELEMENTARY    METEOROLOGY. 

probably  much  more  common  than  we  imagine ;  for  we  notice  it  only  when  it 
becomes  conspicuous  by  means  of  clouds  produced  or  shaped  by  it. 

The  formation  of  clouds  in  waves  depends  011  the  variation  of  density, 
and  hence  of  temperature,  produced  by  the  vertical  component  of  their  motion. 
The  vertical  oscillation,  by  which  a  portion  of  air  is  carried  from  the  trough 
to  the  crest  of  the  wave,  may  in  certain  cases  cause  sufficient  cooling  to 
produce  cloudiness.  If  so,  the  cloud  should  increase  on  the  front  of  the  wave 
and  fade  away  on  the  rear.  Condensation  once  begun  in  this  way  may  give 
rise  to  further  cloud  growth  by  arresting  sunshine ;  but  this  whole  process 
calls  for  detailed  observation  before  it  can  be  considered  well  understood. 

•  The  forced  passage  of  the  wind  over  an  obstacle,  such  as  a  mountain-ridge, 
sometimes  throws  the  current  into  standing  waves  for  some  distance  to 
leeward.  In  northwestern  England,  when  a  damp  wind  blows  over  the  Cross 
Fell  range  from  the  east,  a  cloud  is  formed  over  the  ridge  by  the  forced  ascent 
of  the  air,  and  a  second  cloud,  known  as  the  Helm  Bar,  is  formed  in  the 
ascending  part  of  a  standing  wave  of  wind,  a  short  distance  to  leeward. 
Similar  clouds  may  be  expected  in  the  damper  mountainous  parts  of  our 
western  country,  although  they  have  not  yet  been  recognized. 

Much  larger  examples  of  clouds  formed  in  standing  waves  have  been 
described  over  and  to  leeward  or  northwest  of  the  island  of  Ascension,  where 
they  float  in  the  trade  wind  for  fifty  or  more  miles  in  the  ocean.  It  is 
suggested  that  even  lofty  cirrus  clouds  may  be  formed  by  the  ascent  and 
undulation  of  high  currents  where  their  even  flow  is  disturbed  by  the  arching 
of  the  lower  currents  over  islands  or  mountains ;  but,  like  many  other  sugges- 
tions in  this  chapter,  much  more  observation  is  needed  for  the  full  confirmation 
of  this  process. 

204.  Clouds  do  not  always  float  with  the  air  currents.  It  is  commonly 
assumed  that  the  movement  of  clouds  gives  a  direct  indication  of  the  move- 
ment of  the  air  in  which  they  are  suspended  ;  but  a  number  of  examples 
described  above  show  that  this  is  not  necessarily  the  case.  The  apparently 
fixed,  level  base  of  a  cumulus  cloud  is  really  the  site  of  a  comparatively  active 
ascending  current.  The  stationary  cloud-banners  that  sometimes  stream  out 
t«»  leeward  of  mountain  peaks  merely  indicate  the  space  within  whieh  the 
moving  air  is  reduced  to  a  temperature  below  its  dew-point;  the  air  flows 
rapidly  along,  bearing  the  individual  cloud  particles  with  it,  but  the  cloud 
stands  still.  The  same  is  true  of  clouds  formed  in  fixed  waves,  determined  by 
irregularities  of  the  land.  Finally,  the  movement  of  those  high-level  clouds 
that  seem  to  be  formed  in  the  rippling  wave  crest  of  adjacent  air  currents 
must  differ  by  a  certain  unknown  amount  from  the  motion  of  the  currents  that 
produce  them.  A  later  section  (L'TL'j  will  give  illustration  of  a  cloud  whose 
change  of  outline  actually  progresses  against  the  movement  of  the  wind  by 


DEW,    FllOST    AND    CLOUDS.  173 

which  its  particles  are  carried.  These  facts  should  be  born  in  mind  and 
allowed  for  as  far  as  possible  when  observations  of  clouds  are  employed  in  the 
study  of  the  movements  of  the  atmosphere. 

The  striation  of  lofty  fibrous  clouds  seldom  indicates  the  direction  of  their 
movement  with  respect  to  the  earth's  surface.  They  frequently  drift  trans- 
versely to  their  length  ;  and  then  their  trend  as  well  as  their  drift  should  be 
recorded.  The  twisted  wisps  often  seen  on  the  under  surface  of  lofty  clouds 
are  generally  due  to  the  sinking  of  cloud  particles  from  one  current  into 
another  of  different  direction  and  velocity ;  the  direction  of  the  wisps  then 
indicates  the  movement  of  the  lower  current  with  reference  to  the  upper 
current ;  just  as  the  smoke  from  a  moving  train  is  not  parallel  either  to  the 
railroad  or  to  the  wind,  but  closes  the  triangle  of  which  the  train  and  wind 
movements  are  two  sides. 

205.  Condensation  caused  by  conduction.     It  is  possible  that  certain  thin 
sheets  of  cloud  may  be  formed  in  the  warmer  of  two  adjacent  air-currents 
whose  temperatures  are  distinctly  different.     Conduction  causes  the  tempera- 
ture of  each  current  to  approach  that  of  its  neighbor ;  and  the  cooling  of  the 
warmer  current  may  make  it  cloudy.     Like  several  other  processes  here  con- 
sidered, no  definite  value  can  be  assigned  to  this  one  ;  but  all  possible  processes 
should  be  borne  in  mind  when  undertaking  this  most  difficult  division  of 
meteorological  observation. 

206.  Condensation  by  the  upward  diffusion  of  vapor.     The  occurrence  of 
diurnal  convectional  currents  has  been  explained  as  depending  on  the  over- 
warming  of  the  lower  air  under  sunshine.     It  is  most  marked  on  land  surfaces 
and  in  summer,  when  the  diurnal  range  of  temperature  is  strong.     At  sea, 
where  the  range  may  be  generally  less  than  four  degrees,  the  increase  of 
temperature  in  the  lower  air  does  not  appear  to  be  sufficient  to  cause  instability 
and  convection  ;  and  yet  in  the  trade  belts  and  especially  in  the  doldrums  over 
the  oceans,  local  cumulus  clouds  are  regularly  formed  in  the  morning,  and  rise 
to  great  heights  in  the  afternoon,  generally  causing  rain.    It  may  be  suggested 
that  these  clouds  are  due  in  good  part  to  an  upward  diffusion  or  expansion  of 
water-vapor,  formed  in  excess  at  the  surface  of  the  ocean ;  or  at  least  that  this 
process  is  of  decided  importance  in  combination  with  any  convectional  motion 
that  may  take  place  there.     A  slender  form  of  the  lofty  cumulus  clouds  in  the 
trade  winds  is  said  to  be  characteristic. 

207.  Condensation  by  radiation  from  the  atmosphere.     The  atmosphere 
has  already  been  described  as  a  poor  radiator ;  its  temperature  falls  but  little 
at  night,  because  it  cannot  easily  give  up  its  heat  by  the  emission  of  radiant 
energy.     It  is  therefore  somewhat  uncertain  whether  the  sheets  of   cloud, 


174  ELKMKNTAUY  METEOROLOGY, 

sometimes  observed  in  the  early  morning  after  a  night  that  was  clear  till  a 
late  hour,  can  be  ascribed  to  so  ineffective  a  process  as  the  cooling  of  the  air 
l»v  its  own  radiation.  It  may  be,  however,  that  certain  layers  of  air  are  so 
nearly  at  their  dew-point  in  the  day-time  that  the  little  cooling  of  night  makes 
them  cloudy.  If  the  process  is  once  begun,  its  extension  is  easy ;  for  the  cloud 
particles  themselves  will  cool  by  radiation,  and  the  air  near  them  will  then 
cool  by  conduction  and  radiation  to  them  ;  condensation  once  established  may 
be  rapidly  extended.  A  possible  cause  for  the  moist  condition  of  certain  strata 
of  the  atmosphere  is  found  in  the  outspreading  of  cumulo-stratus  sheets  from 
the  top  of  cumulus  clouds,  as  explained  in  Section  201.  The  cumulo-stratus 
sheet  slowly  dissolves  away  in  the  late  afternoon,  thereby  dampening  the  layer 
of  air  about  it.  Thus  the  transfer  of  vapor  from  the  lower  air  or  even  from 
the  ground  in  the  day-time  may  supply  the  vapor  for  the  formation  of  high- 
level  cloud  sheets  by  radiation  late  in  the  succeeding  night ;  but  like  several 
other  suggestions  regarding  the  origin  of  clouds,  this  is  altogether  problematic. 
It  seems  to  be  physically  possible,  but  its  value  in  actual  occurrence  is 
undetermined. 

208.  Condensation  by  mixture  of  air  currents.  If  two  masses  of  air,  both 
saturated  but  of  unlike  temperatures,  are  thoroughly  mixed,  the  temperature 
of  the  mixture  will  be  below  its  dew-point,  and  a  general  clouding  of  moderate 
density  will  be  the  result.  This  process  of  condensation  was  suggested  in  the 
last  century  by  Hutton,  and  was  for  many  years  regarded  as  the  most  effective 
means  of  producing  clouds  and  rain.  It  is  still  often  referred  to,  but  it  cannot 
now  be  regarded  as  so  important  as  several  of  the  processes  already  considered. 

The  present  understanding  of  meteorological  phenomena  shows  that 
Button's  theory  involves  an  uncommon  process,  and  that  it  is  of  relatively 
little  importance  when  it  occurs,  except  as  a  subordinate  aid  to  other  processes. 
It  is  uncommon,  because  it  seldom  happens  that  two  adjacent  currents  of  unlike 
temperatures  are  both  saturated.  Even  if  this  condition  occurred,  the  intimate 
mixture  of  the  two  currents  is  not  easily  brought  about.  If  mixture  should 
take  place,  the  resulting  condensation  would  not  supply  clouds  and  rainfall  in 
observed  amounts,  unless  improbable  temperatures,  volumes  and  velocities  are 
assumed  for  the  intermixing  currents.  Little  attention  is  therefore  now  given 
to  this  process  alone  as  a  means  of  producing  so  active  a  condensation,  as  to 
cause  rain.  Coupled  with  other  processes,  it  has  some  undetermined  value. 
Smaller  examples  of  its  action  may  perhaps  be  seen  in  the  formation  of  the 
tangled  cirrus  clouds  mentioned  in  Section  202,  although  the  prevalent  dryness 
of  the  upper  air  is  against  such  an  explanation  ;  or  of  the  thin  wave-like  cloud 
films  that  are  sometimes  seen  in  our  southerly  winds  when  they  flow  over 
colder  lower  air;  in  this  case  the  surface  of  contact  is  rer<>^ni/ed  by  the 
wave-like  form  of  the  cloud  film.  In  a  larger  way,  mixture  must  aid  in  the 


DEW,    FROST    AND    CLOUDS.  175 

formation  of  the  great  masses  of  storm  clouds  from  which  most  of  our  winter 
rain  and  snow  falls  ;  but  it  should  be  noticed  that  in  such  cases  the  various 
processes  by  which  the  mixing  currents  have  been  cooled  to  their  dew-points 
continue  in  operation  after  mixture  as  well  as  before,  and  that  the  greater 
amount  of  condensation  is  to  be  expected  from  these  continuous  processes 
rather  than  from  mixture,  whose  cloud-making  ceases  when  the  intermingling 
is  once  accomplished. 

The  process  of  mixture  of  different  air  masses  is  more  often  the  cause  of 
the  dissolution  of  clouds  than  of  their  formation.  When  an  ordinary  morning 
cumulus  cloud  rolls  forward  as  it  rises  and  its  margins  mingle  with  the  higher 
air  that  its  ascending  currents  enter,  the  mixture  of  the  saturated  cloud-bearing 
streams  of  air  with  larger  volumes  of  clear  and  relatively  dry  air  alongside, 
ordinarily  enables  all  the  condensed  vapor  to  dissolve  again.  The  same 
process  must  be  common  in  lofty  cirrus  clouds,  whose  minute  and  thinly- 
scattered  ice  crystals  are  frequently  evaporated  along  the  feathery  margin  of 
the  cloud,  where,  according  to  the  theory  of  cloud-making  by  mixture,  the 
cloud  should  be  densest. 

It  may  be  well  to  refer  briefly  to  another  inefficient  process  often  mentioned 
as  producing  clouds.  This  is  the  ascent  of  warm,  damp  lower  air  into  the 
"  cold  regions  of  the  upper  atmosphere."  It  is  true  that  the  upper  air  is  cold, 
and  that  ascending  currents  become  cloudy,  but  there  is  no  reason  to  think 
that  any  large  part  of  the  cloud  mass  is  due  to  cooling  by  conduction  to  or 
mixture  with  the  cold  and  generally  dry  upper  air  ;  for  if  so,  an  ordinary 
cumulus  would  be  only  a  hollow  shell  of  cloud.  The  ascending  current 
becomes  cloudy  by  reason  of  its  own  mechanical  cooling,  as  has  been  fully 
considered  on  earlier  pages. 

209,  Haze.  There  are  all  gradations  from  a  sky  of  perfect  clearness  to 
turbidity  of  varying  degrees  of  density,  known  as  haze.  This  is  sometimes 
the  product  of  forest  fires,  such  as  are  frequent  on  our  western  mountains  in 
dry  summer  weather,  when  the  transparency  of  the  air  for  hundreds  of  miles 
to  leeward  is  lost  for  weeks  together,  and  only  the  nearer  hills  remain  in  sight. 
In  northern  Europe,  the  smoke  from  the  burning  of  extensive  peat  bogs  some- 
times dulls"  the  sky  over  large  regions.  Haze  is  sometimes  caused  by  the 
presence  of  very  fine  mineral  dust,  gathered  from  deserts  and  suspended  or 
carried  far  away  by  the  winds.  The  islands  west  of  the  Sahara  are  often  thus 
afflicted.  In  our  own  country,  the  warm  southerly  winds  of  spring  and 
summer  are  often  hazy,  with  a  glaring  sky  of  pale  blue ;  the  haze  is  thought 
to  be  largely  composed  of  water  particles,  but  the  cause  of  their  condensation 
is  not  understood.  True  water  vapor  is  entirely  invisible.  Fine  water 
1  (articles  are  more  easily  evaporated  than  larger  ones,  not  only  because  they 
have  small  volume,  but  also  by  reason  of  the  sharp  curvature  of  their  surface ; 


176  ELKMKN TARY    METEOROLOGY. 

it  being  proved  by  experiment  that  evaporation  may  be  continued  from  a 
convex  surface  after  it  ceases  from  a  plane  surface.  The  conditions  of  tin1 
formation  and  duration  of  haze  therefore  still  need  examination.  When  the 
haze  pales  the  blue  of  the  upper  sky,  and  yet  leaves  distant  objects  visible 
through  the  lower  air,  it  is  sometimes  called  cirrus  haze. 

210.  Conditions  that  favor  clear  sky.  It  is  pertinent  to  introduce  in  this 
connection  the  opposite  question  of  the  causes  of  a  clear  or  clearing  sky. 
Probably  the  most  effective  cause  is  a  scarcity  of  water  vapor,  such  as  charac- 
terizes interior  desert  regions,  far  removed  from  oceans  and  enclosed  by  lofty 
mountains.  The  skies  of  such  regions  are  generally  dusty,  but  not  cloudy : 
they  form  the  natural  contrast  to  the  prevailingly  clean  but  cloudy  oceanic 
skies.  The  higher  strata  of  the  atmosphere  are  never  clouded ;  the  quantity 
of  vapor  that  can  exist  there  is  very  small,  and  no  effective  cause  of  condensa- 
tion seems  to  operate  at  great  heights.  Convectional  action  and  rapid  changes 
of  temperature  are,  as  a  rule,  limited  to  the  lower  atmosphere. 

The  occurrence  of  descending  air  currents  is  practically  prohibitive  of 
cloud  making.  It  is  true  that  the  compression  of  a  mass  of  moist  air,  whose 
temperature  is  maintained  at  a  constant  value,  will  soon  cause  condensation  of 
vapor  into  mist ;  but  the  natural  process  of  compression  during  descent  is 
always  accompanied  by  the  generation  of  heat  and  a  consequent  rise  of 
temperature,  which  effectually  counteracts  the  tendency  to  condensation  due 
to  decrease  of  volume  ;  except  in  those  special  cases  where  the  descent  becomes 
very  slow,  as  on  approaching  the  ground  in  winter,  and  where  the  heat  gained 
by  compression  is  lost  by  radiation  and  conduction  to  the  cold  surface  of  the 
earth,  producing  heavy  fogs  in  cold  weather  (Sect.  249).  The  trade  winds,  or 
the  northerly  winds  that  follow  our  spells  of  cloudy  and  rainy  weather  (Sects. 
294,  315),  approach  the  equator  and  gain  a  higher  temperature  which  effectually 
dissipates  any  clouds  that  they  may  at  first  have  borne ;  and  such  winds  are 
prevailingly  clear,  unless  prompted  to  roll  over  by  local  convection  or  irregu- 
larity of  path.  The  flow  of  a  cool  wind,  bearing  fog  from  an  ocean  upon  a 
warm  summer  land,  soon  brings  about  a  sufficient  rise  of  temperature  to 
evaporate  the  fog.  Quiet  air,  or  air  without  vertical  components  of  motion, 
allows  cloud  particles  to  settle  slowly  to  levels  of  higher  temperature,  and 
therefore  soon  becomes  clear ;  as  on  calm  summer  evenings,  when  the  clouds 
produced  by  convectional  action  during  the  hotter  hours  of  the  day  sink  down 
and  fade  away,  leaving  a  clear,  star-lit  sky.  Conduction  between  adjacent  air 
currents,  appealed  to  already  to  produce  cloud  sheets,  may  under  favorable 
conditions  cause  them  to  disappear;  for  if  a  sheet  of  cloud,  borne  by  a  c«»ol 
current,  comes  in  contact  with  a  warm  and  dry  stratum  of  air,  the  warmth 
gained  by  the  former  from  the  latter  may  dissolve  the  cloud  away  and  leave? 
both  currents  clear.  The  mixture  of  two  air  masses,  one  of  which  is  cloudy, 


DEW,    FROST    AND    CLOUDS.  177 

is  quite  as  likely  to  produce  clear  air  as  to  increase  the  cloudiness  ;  the 
disappearance  of  clouds  on  the  rear  of  our  stormy  areas  when  fair  weather  is 
approaching  may  be  in  good  part  due  to  this  process. 

211.  Classification  of  clouds.  The  previous  sections  have  described  the 
forms  of  clouds  produced  in  different  ways.  The  descriptions  have,  however, 
involved  certain  hypothetical  explanations,  which  may  not  in  all  cases  be 
correct ;  these  may  serve  as  suggestions  for  deliberate  investigation,  but  they 
are  not  serviceable  as  guides  in  recording  the  occurrence  of  clouds  at  the  hours 
of  regular  observation.  It  is  therefore  advisable  to  classify  clouds  in  some 
simple  manner  for  convenient  record. 

The  classification  adopted  by  the  Signal  Service  in  this  country,  and  still 
in  use  in  the  Weather  Bureau  (Sect.  318),  is  slightly  modified  from  that  of 
Howard,  proposed  in  1803.  A  somewhat  different  system  was  recommended 
by  the  International  Meteorological  Congress  held  at  Munich,  in  1891.  The 
following  table,  prepared  by  Mr.  H.  H.  Clayton,  indicates  the  relations  of  the 
two  systems  :  — 

WEATHER  BUREAU.  INTERNATIONAL  CONGRESS. 

Cirrus  Cirrus 

Cirro-stratus  Cirro-stratus  (alto-stratus) 

(  Cirro-cumulus 
Cirro-cumulus  •<   . 

(  Alto-cumulus 

Cumulus  Cumulus 

(  Strato-cumulus 
Cumulo-stratus  -}  _, 

(  Cumulo-nimbus 

Nimbus  Nimbus 

Stratus  Stratus 

The  following  descriptions  apply  to  the  terms  adopted  by  the  International 
Congress.  It  is  very  desirable  that  uniformity  the  world  over  should  be 
gained  in  terms  of  this  kind.  Comparison  of  observations  is  otherwise 
impossible. 

Cirrus  clouds  consist  of  slender  fibres,  sometimes  in  long  parallel  lines, 
sometimes  in  feathery,  curled,  tangled  or  clotted  arrangement.  Their  form 
changes  slowly  and  their  movement  is  apparently  not  so  fast  as  that  of  clouds 
at  lower  levels  ;  but  as  their  altitude  varies  commonly  between  five  and  ten 
miles  above  sea-level,  their  actual  velocities  may  be  rapid,  from  50  to  100  or 
200  miles  an  hour.  Cirrus  clouds,  as  a  rule,  drift  eastward  (Sect.  147)  ;  but 
they  occasionally  advance  slowly  to  the  west  in  connection  with  storms. 

Cirro-stratus  clouds.  True  cirrus  fibers  are  often  associated  with  horizontal 
cloud  layers  at  similar  or  slightly  less  altitudes,  as  if  formed  by  the  matting 
together  of  growing  filaments.  These  layers  are  often  extended  in  bands  of 
considerable  length,  sometimes  in  parallel  trains,  straight  or  gently  curved, 
and  associated  with  true  cirrus  fibres;  they  may  reach  all  across  the  sky  and 


178  KLEMENTAKV    MKTKOlloLOCY. 

from  perspective  seem  to  converge  at  nearly  opposite  points  of  the  horizon, 
being  then  called  polar  bands  or  "Noah's  Ark."  The  layers  are  often  more 
or  less  fibrous,  striated,  or  rippled  ;  they  frequently  show  a  tendency  to  break 
up  into  separate  clots  (cirro-cumulus).  Cirro-stratus  is  sometimes  defined  as 
the  product  of  cirrus  sinking  to  a  lower  level  ;  but  it  is  questionable  whether 
such  descent  takes  place.  Cirrus  haze  is  applied  to  a  thin  and  even  over- 
casting of  the  sky  at  high  levels,  below  which  various  other  clouds  may  float. 
Cirro-stratus  is  applied  to  layers  of  distinctly  greater  density  ;  when  heavier 
and  lower,  they  are  called  alto-stratus.  If  not  too  dense,  the  cirro-stratus 
and  alto-stratus  produce  halos  around  the  sun  or  moon  ;  and  from  this,  as  well 
as  from  the  great  altitude  of  their  occurrence,  they  are  known  to  be  composed 
of  ice  crystals  and  not  of  water  drops. 

Cirro-cumulus.  The  separate  clots  or  cloud  balls  into  which  lofty  cloud 
luvt-rs  often  break  up  come  under  this  division.  They  often  closely  resemble 
the  form  taken  by  the  foam  in  the  eddying  wake  of  a  steamer.  When  well 
defined  and  closely  grouped,  they  are  called  mackerel  clouds.  Cirro-cumulus 
may  also  be  applied  to  relatively  isolated  clouds  of  moderate  size  and  consider- 
able altitude,  from  which  long  cirrus  streamers  settle  down  to  lower  levels, 
often  twisting  as  they  enter  currents  of  different  direction  from  the  one  in 
which  the  supplying  cloud  is  carried.  These  are  in  reality  little  snow-flurries 
in  the  upper  air ;  the  long,  trailing  fall  of  snow  sometimes  remains  after  the 
cloud  at  its  source  has  ceased  to  act  and  has  dissolved  away. 

Cumulus.  This  form  of  cloud  has  already  been  so  fully  described 
that  little  need  be  said  of  it  here.  Its  flatter  forms,  often  grouped  closely 
together  so  as  nearly  to  overcast  the  sky  and  common  in  fair  windy  weather, 
are  called  strato-cumulus.  Its  still  lower  ragged  forms,  often  assumed  during 
the  early  stages  of  cloud  growth  and  in  storms,  are  called  fracto-cumulus. 
Higher  smaller  forms  are  named  alto-cumulus. 

Cumulo-nimbus  is  the  name  given  to  the  large  overgrown  cumulus  clouds 
that  have  reached  the  dimensions  of  thunder  storms,  having  above  the  "  thunder- 
heads  "  an  outflow  of  alto-stratus  or  cirro-stratus,  with  a  fibrous  margin  some- 
times called  "  false  cirrus."  The  under  surface  of  these  extended  overflows 
from  cumulo-nimbus  clouds  is  sometimes  curiously  festooned,  where  the  filmy 
cloud  layers  settle  slowly  to  lower  levels. 

Stratus.  This  name  was  originally  applied  to  low-lying  fogs,  such  as  form 
at  night  or  in  cold,  quiet  winter  weather  on  lowlands  or  in  valleys  ;  it  has  been 
extended  to  include  low  foggy  cloud  sheets  floating  overhead,  but  with  the  base 
at  a  moderate  height.  It  should  not  be  applied  to  thin  cloud  sheets  commonly 
seen  at  sunset  at  a  great  altitude  ;  these  being  either  fading  strato-cumulus  or 
alto-stratus.  When  lying  on  the  earth,  tin-  stratus  cloud  is  simply  called  fog. 
Nimbus.  Any  extended  cloud  from  which  rain  or  snow  is  falling  is 
commonly  called  nimbus  ;  it  is  generally  preceded  by  stratus,  and  still  earlier 


DEW,    FROST    AND    CLOUDS. 


179 


by  alto-stratus.  As  this  refers  rather  to  the  state  of  the  weather  than  the 
kind  of  cloud,  the  term  is  not  entirely  satisfactory.  An  observer  on  a  high 
hill,  receiving  rain  from  a  dripping  cloud  not  far  overhead,  would  call  it 
nimbus ;  while  an  observer  on  an  adjacent  lowland  might  call  the  same  cloud 
by  some  other  name.  A  heavy  cloud  sheet,  hanging  at  a  moderate  height  and 
threatening  rain,  would  be  called  stratus  by  an  observer  where  rain  had  not 
begun  ;  and  nimbus  by  another  a  few  miles  away  where  rain  was  already  falling. 
It  is  often  preferable  to  employ  some  indefinite  term,  as  "  overcast,"  when  the 
cloud  evenly  covers  the  whole  sky  and  its  nature  cannot  be  determined. 

212.  Altitude  of  clouds.  Simultaneous  observations  of  the  angular  altitude 
and  azimuth  or  direction  of  clouds  made  by  two  observers  communicating  with 
each  other  by  telephone  from  stations  a  mile  or  more  apart  serve  to  determine 
the  height  at  which  the  clouds  float,  their  dimensions,  and  the  direction  and 
velocity  of  their  motion.  Simultaneous  photographs  of  clouds  from  different 
stations  have  been  used  in  the  same  way.  This  style  of  observation  might  be 
taken  up  to  advantage  by  observers  who  can  provide  themselves  with  suitable 
instruments  for  angular  measurements,  and  can  use  a  telephone  connection 
between  their  stations. 

The  following  series  of  altitudes  (in  meters)  have  been  determined  by 
recent  measurements  in  Sweden,  and  at  the  Blue  Hill  Observatory  near 
Boston ;  the  comparative  table  being  prepared  by  Mr.  Clayton,  observer  at 
the  last  named  station.1 


KIND  OF  CLOUD. 

BLUE  HILL,  MASSACHUSETTS. 

SUMMER  HEIGHT. 

WINTER  HEIGHT. 

Mean. 

Max. 

Min. 

Mean. 

Max. 

Min. 

Meters. 
9923 
8754 
6481 
7606 
6406 
3168 
2003 
8242 

Meters. 
14930 
12134 
12050 
10520 
8204 
7047 
3328 
12360 

Meters. 
5392 
5521 
2290 
4772 
3119 
784 
1109 
5392 

Meters. 
8051 
7846 
2930 
6992 

Meters. 
11560 
8512 

Meterg. 
3764 
6823 

Low  Cirro-stratus               

8570 

4571 

High  Alto-cumulus  
Low  Alto-cumulus  

2884 

'•False  Cirrus" 

Cumulo-nimbus  (base)       
Cumulus  (top)     

1202 
2181 
1473 
712 
583 

1590 

3582 
1720 
2050 

884 
1455 
601 
65 
120 

1552 

2058 

1046 

Cumulus  (base)                       ... 

1381 

2690 

532 

\imbus 

^tratus 

503 

See  Annals  Harvard  College  Observatory,  XXX,  1892,  262. 


180 


ELEMENTARY   METEOROLOGY. 


KIND  OF  CLOUD. 

UPSALA,  SWEDEN. 

STORLIEN,  SWEDEN. 

SUMMER  HEIGHT. 

SUMMER  HEIGHT. 

Mean. 

Max. 

Min. 

Mean. 

Max. 

Min. 

Meters. 
8878 
9254 
5198 
6465 
5586 
2771 
2331 
3897 
2848 
1405 
1855 
1386 
1527 
623 

Meters. 
13376 
11391 
5657 
10235 
8297 
3820 
4324 
5470 
5970 
1630 
3611 
2143 
3700 
994 

Meters. 
4970 
6840 
4740 
3880 
4004 
1498 
887 
2465 
1400 
1180 
900 
743 
213 
414 

Meters. 
8271 

Meters. 
10419 

Meters. 
6148 

Low  Cirro-stratus    •      •  •  • 

6337 
4562 
2744 
1788 

7358 
4918 
3844 
2830 

6283 

4174 
1182 
638 

"False  Cirrus"   

2504 

3515 

2998 

2181 
1401 
1664 
998 

2997 
1901 
5741 

1140 
929 
017 

Stratus  

The  prevalence  of  the  different  cloud  forms  at  certain  altitudes  is  more 
clearly  brought  out  by  the  following  table  from  the  same  source  as  the 
preceding  one. 

MEAN  HEIGHTS  AND  VELOCITIES  OF  DIFFERENT  CLOUD  FORMS. 


BLUE  HILL,  MASS. 
(CLAYTON  &  FERGUSSON.) 


half 

year, 

Winter 

half 
year, 


Height  (meters) 
Velocity  (met- 


r  Cirrus  Level. 
9757 


eMHei 

.    J  Vel 


Height 8012 

Velocity 43.9 


Year. 


(Height 8884 

\  Velocity 35.9 


2°  Cirro- 
cumulus. 

8228 
24.1 

6039 
40.9 

6633 
32.6 


3°  Alto- 
cumulus. 

4228 
11.2 


3484 
20.2 

3856 
15.7 


4°  Cum- 
ulus.    5"  Stratus 


Summery 

half     ^Height 

year.    J 
Winter  ^ 

half      ^Height 

year.    J 

Year.     * 


BERLIN,  GERMANY. 

(VETTIN.) 


7220 

}  Velocity 17.1 


4520 


3670 

4020 
14.0 


1976 

2260 
11.7 


1657 
8.9 

1571 
13.7 

1614 
11.3 


1310 


1000 

1190 
10.7 


56:3 
7.2 

454 
10.2 

508 

8.7 


645 


440 

4!>0 
10.2 


DEW,    FROST    AND    CLOUDS.  181 

Although  the  neights  of  the  several  cloud  levels  vary  from  summer  to 
winter,  the  ratios  of  the  successive  heights  are  essentially  constant  for  the 
Y'-ar.  as  appears  from  the  following  numbers  :  the  height  of  the  stratus  level 
being  taken  as  unity  for  each  half  year. 

Level  1.     2.  3.  4.  5. 

Ratios  of  cloud  levels  <  Summer     ...       1          3.0          7.5        15.0        17.3 
at  Blue  Hill,  Mass.  |  Winter .     ...       1          3.4          7.7        11.1        17.6 

The  height  attained  in  summer,  especially  by  clouds  of  middle  and  upper 
levels,  is  generally  greater  than  in  winter. 

213,  Observations  of  clouds.  Weather  records  should  include  a  statement 
of  the  kind  and  quantity  of  clouds  seen  at  the  usual  hours  of  observation,  with 
their  direction  and  relative  velocity  of  movement.  If  it  is  desired  to  determine 
simply  the  relative  frequency  of  clear  and  cloudy  weather,  little  trouble  need 
be  taken  to  classify  the  clouds,  as  their  nomenclature  is  a  matter  of  difficulty 
because  of  the  frequent  occurrence  of  forms  which  the  observer  cannot  surely 
refer  to  any  class  ;  but  if  the  observer  wishes  to  learn  something  of  atmospheric 
processes  for  himself,  he  should  give  at  least  as  much  time  to  cloud  observations 
as  to  all  the  other  records  together.  The  various  kinds  of  clouds  should  be 
carefully  distinguished;  the  changes  from  one  form- to  another  should  be 
noted,  and  the  relation  of  the  various  forms  to  weather  changes  should 
be  thoroughly  studied  out.  Descriptive  accounts  must  be  often  added  to  the 
brief  terms  by  which  clouds  are  named ;  sketches  and  photographs  are  of  much 
service  in  giving  defmiteness  to  the  record.  Instruction  in  this  subject  is 
difficult  from  the  great  variety  of  cloud  forms ;  it  can  be  best  gained  by 
reference  to  photographs  or  well-executed  plates,  as  verbal  descriptions  are 
generally  insufficient.  It  must  frequently  happen  that  observers  taught  only 
from  books  will  use  different  names  for  clouds  of  the  same  kind. 

The  quantity  of  each  kind  of  cloud  should  be  determined  separately  ;  the 
cloud  area  is  estimated  in  tenths  of  the  sky  occupied  by  the  clouds.  In 
general  descriptions  of  the  weather,  less  than  T3^  is  called  clear  ;  from  T3^  to  T7^, 
fair ;  more  than  T7^,  cloudy  ;  overcast  is  often  used  to  denote  an  even  and 
complete  covering  of  the  sky  by  clouds  of  any  kind.  Cloudless  is  more 
emphatic  than  clear,  and  refers  to  a  sky  in  which  no  clouds  are  seen. 

The  direction  and  velocity  of  cloud  movement  are  commonly  estimated  by 
eye  ;  as  "slow  from  the  XW."  or  "  fast  from  E."  It  is  desirable  that  a  more 
accurate  method  should  be  introduced ;  for  the  direction  of  cloud  movement  is 
remarkably  steady  for  hours  together  and  is  susceptible  of  measurement  within 
a  few  degrees  ;  and  the  velocity  of  cloud  drifting  is  certainly  a  very  important 
element  in  weather  changes.  The  direction  is  best  determined  by  noting  the 
path  of  the  reflection  of  the  cloud  in  a  horizontal  mirror,  at  which  the  observe? 


182  ELEMENTARY    METEOROLOGY. 

looks  through  an  eye-piece  that  remains  fixed  during  the  observation.  If  the 
eye-piece  is  placed  so  that  the  reflection  of  a  certain  part  of  the  cloud  falls  at 
the  center  of  the  mirror,  and  after  a  few  seconds  a  radial  arm  is  turned  so  as 
to  bring  its  edge  on  the  position  then  taken  by  the  cloud,  the  edge  of  the  arm 
will  lie  parallel  to  the  cloud's  motion,  on  the  admissible  assumption  that  the 
cloud  is  drifting  in  a  horizontal  plane.  After  setting  the  arm,  it  is  well  to 
wait  again  a  few  seconds  to  see  if  the  cloud  reflection  travels  along  the  line 
thus  marked.  Without  such  instrumental  aid,  the  direction  of  clouds  under 
«JO°  altitude  cannot  be  safely  taken  ;  but  with  a  mirror  the  directions  can  be 
well  determined  even  down  to  ten  degrees  from  the  horizon. 

If  the  eye-piece  through  which  the  observer  looks  at  the  cloud  reflection  is 
always  held  at  a  certain  height  over  the  mirror,  then  the  relative  velocity  of 
the  cloud  drifting  can  be  measured  by  counting  the  number  of  divisions  on  the 
radial  arm  passed  over  by  the  cloud  in  a  given  time,  as  ten  seconds.  This 
provides  a  simple  and  uniform  scale  for  record,  much  more  closely  comparable 
•at  different  times  and  places  than  mere  estimate  by  the  unaided  eye. 

Assuming  that  each  cloud  of  a  class  lies  at  the  level  determined  for  others 
of  its  kind,  the  estimates  of  velocity  here  described  can  easily  be  reduced  to 
actual  velocities.  Measurements  of  this  kind  are  strongly  recommended  to 
interested  observers. 

In  studying  the  movement  of  clouds,  it  is  desirable  to  discriminate  between 
the  generally  slow  structural  movement  of  one  part  of  the  cloud  with  respect 
to  the  rest,  and  the  more  rapid  bodily  drifting  of  the  whole  cloud  in  the  air 
currents.  Ordinary  records  refer  only  to  the  latter  movement.  It  is  important 
to  note  also  the  manner  in  which  the  margin  or  filaments  of  a  cloud  grow  or 
dissolve  ;  and  the  process  by  which  a  cloud  changes  its  form  from  one  class  to 
another.  The  suggestions  of  Section  204  should  be  borne  in  mind  in  this 
connection. 

214.  Sunshine  records.  An  automatic  record  of  the  amount  of  sunshine 
—  the  converse  of  the  amount  of  cloudiness  —  is  obtained  by  various  instruments. 
Some  employ  a  sphere  of  glass  by  which  the  sun's  rays  are  focused  on  a 
curved  sheet  of  paper  at  all  hours  of  the  day  ;  others  secure  a  photographic 
print  of  the  track  of  a  fine  solar  ray  that  falls  through  a  minute  aperture  on 
sensitive  paper.  In  all  cases,  even  a  faint  cloudiness  over  the  sun  may  be 
recognized  by  a  weakening  of  the  record,  and  heavy  clouds  covering  the  sun 
prevent  any  record  being  made.  The  hours  of  sunshine  for  a  month,  divided 
by  the  total  number  of  day-time  hours  in  the  month,  give  an  indication  of  the 
prevalence  of  clear  or  cloudy  weather. 


CYCLONIC    STORMS    AND    WINDS.  183 

CHAPTER   X. 

CYCLONIC    STORMS    AND    WINDS. 

215.  Unperiodic  winds.     In  all  that  has  preceded,  the  reader  will  find  no 
explanation  of  the  frequent  irregular  changes  of  wind  and  weather.      The 
explanations  thus  far  given  account  for  the  diurnal  warming  and  cooling  of 
the  air,  for  the  progressive  change  from  winter  to  summer,  for  the  gradual 
variation  of  our  prevailing  winds  from  southwest  in  summer  to  northwest  in 
winter,  for  their  greater  velocity  by  day  and  their  falling  off  nearly  to  a  calm 
at  night,  for  the  inflow  and  outflow  by  day  and  night  on  coast  lines,  and  for 
the  regular  diurnal  up  and  down  currents  in  mountain  valleys  ;  but  all  this 
gives  no  mention  of  the  shifting  of  the  wind  from  one  direction  to  another  as 
spells  of  cloudy  and  rainy  weather  pass  by,  leaving  the  sky  clear  behind  them. 
Some  additional  explanation  must  be  found  for  the  southerly  winds  of  winter 
that  spring  up  after  a  time  of  calm,  bringing  cloudy  skies  and  rain,  often 
increasing  in  velocity  after  sunset,  and  even  causing  a  rise  of  temperature 
during  the  night ;  followed  perhaps  the  next  morning  by  winds  veering  to  the 
west  and  northwest,  when  the  sky  clears  and  the  temperature  rapidly  falls, 
even  without  the  customary  rise  at  noon.    These  quickly-shifting  winds  belong 
with  the  stormy  disturbances  of  the  general  circulation,  to  be  considered  in 
this  chapter.     As  they  spring  up  at  irregular  intervals,  being  brief  and  light 
or  longer  and  stronger  as  may  happen,  they  were  referred  to  a  group  of  stormy 
winds  in  the  classification  already  given  in  Section  138. 

216,  Cyclones,  thunder  storms,  and  tornadoes.     Winds  of  this  group  are 
intended  to  include  all  those  whose  causes  cannot  be  clearly  referred  to  some 
periodic  change  of  temperature,  and  which  are  yet  certainly  dependent  directly 
or  indirectly  on  differences  of  atmospheric  temperature  controlled  by  the  sun. 
We  may  now  subdivide  them,  ^taking  as  the  first  class  those  large  disturbances 
so  commonly  shown  on  the  weather  maps  as  areas  of  low  barometric  pressure, 
500  or  1000  miles  in  diameter,  accompanied  by  shifting  winds,  great  areas  of 
cloud  and  smaller  areas  of  rain  or  snow  ;  these  will  be  called  cyclones  l  or 
cyclonic    storms.      The    second   class    includes    those    smaller   disturbances, 
consisting   of   cloud  masses,  ten   or   a   hundred  miles    in   length,  advancing 
broadside  or  obliquely  across  the  country,  giving  forth  drenching  rain,  and 
known  as  thunder  storms  from  their  electric  display.     A  third  class  includes 
the   violent   whirlwinds    of   excessive    destructive   force,   accompanied   by   a 

1  See  note  to  Section  266. 


184  ELEMENTARY    METEOROLOGY. 

pendent  funnel  cloud  from  a  much  greater  cloud  mass  above ;  these,  although 
commonly  called  cyclones  in  this  country,  will  here  be  referred  to  as  tornadoes. 
Cyclones,  thunder  storms,  and  tornadoes,  when  fully  developed,  all  possess 
active  or  violent  winds,  and  are  therefore  often  referred  to  under  the  general 
word,  storms.  They  all  develop  clouds  so  rapidly  that  rain  or  snow  falls  from 
them,  thus  causing  much  more  benefit  than  the  occasional  harm  that  is  caused 
by  their  winds.  All  these  irregular  winds  possess  the  common  feature  of 
progression  as  a  whole  from  place  to  place,  being  in  this  respect  unlike  all 
other  classes  of  winds. 

Beginning  with  the  largest  of  these  disturbances,  it  should  be  noted  that 
their  more  characteristic  examples,  which  will  be  called  cyclones  or  cyclonic 
storms,  may  be  associated  in  a  graded  series  with  disturbances  of  less  and  less 
violence  until  the  distinguishing  features  of  their  class  are  hardly  perceptible. 
Their  winds  may  be  gentle,  their  central  low  pressure  faintly  developed,  their 
cloud  area  small  and  their  rainfall  practically  wanting ;  and  yet  under  the 
various  names  of  areas  of  low  pressure,  barometric  depressions,  and  barometric 
minima,  all  these  weak  disturbances  should  be  classified  with  the  distinct 
cyclonic  storms  in  which  the  same  features  are  more  fully  developed.  In 
this  chapter,  however,  only  the  stronger  examples  will  be  considered  ;  leaving 
some  mention  of  the  others  for  the  chapter  on  Weather. 

It  is  advisable  to  divide  cyclones  into  two  subordinate  classes,  and  to 
consider,  first,  those  which  originate  in  the  torrid  zone  near  but  not  over  the 
equator,  and  which  are  therefore  commonly  called  tropical  cyclones ; l  second, 
those  which  are  first  seen  in  the  temperate  or  frigid  zones,  and  which  may 
therefore  be  called  extra-tropical  cyclones  or  cyclonic  storms.  These  two 
classes  are  closely  alike  in  many  respects,  and  when  tropical  cyclones  move 
poleward  along  a  curved  path  into  one  temperate  zone  or  the  other,  as  is  their 
habit,  they  cannot  be  distinguished  from  the  stronger  examples  of  the  extra- 
tropical  cyclones  ;  but  the  two  classes  are  unlike  in  certain  striking  features, 
as  well  as  in  the  conditions  of  their  formation,  and  good  reason  will  appear  for 
referring  them  to  different  conditions  of  origin. 

TROPICAL  CYCLONES. 

217.  Tropical  cyclones  are  vast  whirlwinds,  from  one  to  three  hundred 
miles  or  more  in  diameter,  whose  spiral  inflowing  currents  attain  destructive 
violence  near  the  center,  turning  systematically  to  the  left  in  the  northern 
hemisphere  and  to  the  right  in  the  southern,  around  a  centra)  area  of  Ion- 
pressure.  They  are  accompanied  by  ^ivat  sheets  and  masses  of  clouds,  from 
which  rain  falls  in  torrents,  while  h.n^  cirrus  plumes  spread  out,  above  m,  ;,H 
sides.  In  the  center  of  the  whirl,  where  the  pressure  is  lowest,  the  wind  falls 

1  Properly  "  inter-tropical  cyclones  "  ;  but  the  briefer  term  is  commonly  employed. 


CYCLONIC    STORMS    AND    WINDS.  185 

away,  leaving  a  calm,  and  here  the  rain  ceases  and  the  clouds  may  dissolve, 
showing  a  clear  sky  overhead ;  this  central  space  is  therefore  called  the  "  eye 
of  the  storm."  The  whole  cyclone  thus  constituted  moves  slowly  along  a 
ct'itain  rather  well-defined  path  or  track  obliquely  westward  and  poleward 
over  the  ocean  from  its  sub-equatorial  beginning  towards  the  temperate  zone, 
gradually  turning  to  an  oblique  eastward  and  poleward  motion  at  latitude  25° 
or  30°.  If  land  is  encountered,  the  storm  weakens,  and  often  dies  away.  The 
appearance  of  a  tropical  cyclone  at  sea  may  be  described  as  follows. 

218.  Approach  and  passage  of  a  tropical  cyclone.  In  the  torrid  zone,  the 
ordinary  succession  of  weather  from  day  to  day  is  remarkably  constant.  The 
range  of  temperature,  the  faint  double  oscillation  of  the  barometer,  the  periodic 
increase  and  decrease  of  cloudiness  all  show  a  regularity  of  recurrence  that  is 
unknown  in  our  latitudes.  If  in  such  a  region  the  barometer  is  noticed  to  rise 
unusually  high,  or  to  stand  stationary  when  its  diurnal  fall  is  expected,  this 
may  be  often  on  land  the  first  sign  of  a  coming  cyclone ;  but  at  sea,  the  faint 
rise  of  the  barometer  is  preceded  by  the  arrival  of  a  long  rolling  swell  that 
swings  rapidly  out  from  the  storm  on  all  sides,  so  as  to  herald  its  coming  even 
three  or  four  days  before  its  arrival. 

The  faint  rise  of  the  barometer  is  felt  on  nearly  all  sides  of  the  storm  area, 
and  it  therefore  marks  what  may  be  called  the  pericyclonic  ring.1  When  its 
highest  pressure  is  reached,  the  wind  commonly  fails.  Then  fine  plumiform 
cirrus  clouds  are  seen  spreading  over  the  sky  from  the  quarter  towards  the 
storm  center,  which  may  then  be  one  or  two  hundred  miles  away  in  the 
direction  of  the  doldrums ;  and  about  the  time  of  the  appearance  of  these 
clouds  the  barometer  slowly  falls  and  the  calm  is  succeeded  by  a  gentle  breeze. 
The  air  becomes  sultry,  and  the  sunsets  take  on  lurid  colors.  When  first  felt, 
the  breeze  generally  blows  five  or  six  points  to  the  right  (in  the  northern 
hemisphere)  of  the  direction  leading  to  the  storm  center.  All  these  signs 
become  more  marked  as  the  cyclone  draws  near ;  the  cirrus  clouds  thicken  and 
become  matted  together  in  cirro-stratus  form,  veiling  the  blue  of  the  sky ;  the 
refraction  of  sunlight  through  the  ice  crystals  of  the  clouds  forms  halos  around 
the  sun  or  moon,  with  the  orange  or  red  color  on  the  inner  and  the  blue  on  the 
outer  side  of  the  circle.  Later,  the  mass  of  clouds  becomes  so  thick  as  to 
obscure  the  sun,  and  leave  the  upper  air  evenly  overcast.  The  winds  have 
freshened  by  this  time,  and  blow  to  the  right  of  a  low  and  distant  mass  of 
dark  cloud ;  isolated  patches  of  cloud  are  seen  to  form  at  one  side,  increase  in 
size,  and  flow  in  to  join  the  central  nimbus  mass.  The  wind  increases  to  a 
gale,  the  waves  rise  on  the  sea,  the  dark  clouds  approach,  thickening  as  they 
come,  and  rain  begins  to  fall  from  them.  The  storm  center  may  be  then  fifty 
or  more  miles  away,  advancing  slowly  with  the  whole  system  of  whirl:\ig 

1  See  American  Meteorological  Journal,  July,  1886. 


186  ELEMENTARY    METEOROLOGY. 

winds  at  a  rate  of  eight,  ten,  or  twelve  miles  an  hour.  The  barometer 
continually  sinking,  and  at  last  falls  rapidly;  with  this  the  roaring  wind 
increases  to  full  hurricane  strength,  the  low  scud  clouds  fly  before  its  blasts, 
the  lightning  flashes,  the  rain  descends  in  drenching  torrents,  cooling  the 
sultry  air.  All  the  elements  are  in  uproar;  yet  only  a  day  or  two  before 
there  may  have  been  no  sign  of  the  coming  storm,  except  the  ominous  heaving 
of  the  sea. 

Before  the  law  of  storms  was  learned,  many  a  ship  was  borne  before  such 
a  hurricane,  with  all  sails  furled  or  blown  away,  helpless  in  the  violence  of 
the  winds  and  waves  ;  and  when  the  vessel  was  at  last  about  to  founder,  the 
wind  has  suddenly  weakened  to  a  calm  in  the  eye  of  the  storm  ;  falling  from  its 
greatest  violence  to  an  almost  perfect  repose  in  fifteen  minutes  or  less.  The 
rain  ceases,  even  the  clouds  may  break  away,  showing  the  blue  sky  by  day  and 
the  stars  by  night ;  but  the  waves  still  roll  and  toss,  and  in  even  more  dreaded 
form  than  in  their  regular  heaving  before  the  hurricane  ;  for  in  the  eye  of  the 
storm  they  swing  in  from  all  sides,  and  pitch  and  heave  tumultuously,  forming 
irregular  pits  and  peaks  of  water  which  strain  a  vessel  violently,  even  to 
leaking  and  sinking.  A  few  careful  records  made  in  the  calm  storm  center 
while  passing  over  a  land  station  show  a  peculiar  change  in  the  temperature 
and  humidity  of  the  air.  Underneath  the  surrounding  heavy  clouds,  the  air  is 
somewhat  cooled  and  held  close  to  its  dew-point  by  the  rainfall ;  yet  the  air 
within  the  calm  center  has  been  found  to  be  comparatively  dry  with  a 
temperature  unduly  high  ;  but  it  is  not  yet  known  if  these  features  always 
prevail.  The  diameter  of  the  calm  space  may  be  ten,  twenty,  or  thirty  miles, 
perhaps  a  tenth  or  a  fifteenth  of  the  diameter  of  the  whole  storm  ;  and  its 
duration  in  passing  a  given  point  may  vary  from  half  an  hour  to  two  hours  : 
the  barometer  reading  in  the  center  may  be  even  less  than  27  inches. 

As  the  hurricane  on  the  further  side  of  the  central  calm  approaches  the 
observer,  its  moaning  can  be  heard  in  the  distance,  rising  to  a  portentous  roar 
as  it  comes  near,  and  then  breaking  suddenly  with  as  great  fury  as  the 
hurricane  which  died  away  before,  but  its  direction  is  now  the  reverse  of  that 
of  the  winds  by  which  the  calm  was  preceded.  All  the  elements  of  the 
cyclone  now  re-appear ;  the  blasts  of  the  wind  beat  up  the  waves  to  their 
greatest  height,  the  clouds  hang  low  and  heavy  over  the  darkened  sea,  the  rain 
fulls  again  in  torrents ;  and  then  as  the  storm  gradually  moves  away,  all  these 
signs  of  its  activity  weaken.  In  the  course  of  a  day  or  two,  the  barometer  rises 
ii.-arly  to  its  usual  height,  the  wind  dies  down,  the  waves  fall  to  a  long  low 
swell,  the  lower  clouds  recede,  the  lofty  cirrus  plumes  retreat  after  them,  and 
the  sky  is  left  in  its  accustomed  clearness. 

219.  Law  of  storms.  Until  about  1830,  there  was  little  knowledge  of  the 
systematic  courses  followed  by  the  winds  in  cyclones,  and  ships  at  sea  were  at 


CYCLONIC    STORMS    AND    WINDS. 


187 


the  mercy  of  every  storm.  Near  the  beginning  of  this  century,  Capper  of  the 
British  East  India  Company  had  announced  that  the  storms  of  the  Bay  of 
Bengal  were  vast  whirlwinds  ;  and  about  1826,  Brandes  in  Germany  approached 
an  understanding  of  the  general  bad-weather  storms  of  that  country.  A  few 
years  later,  Dove  in  Germany  (1828),  and  soon  after  Redfield  in  this  country 
(1831),  gave  full  demonstration  of  the  systematic  rotation  of  storm  winds,  and 
of  the  regular  progression  of  the  whole  disturbance  from  place  to  place.  These 
early  investigators  employed  a  method  of  study  that  has  since  then  been 
generally  introduced  in  preparing  our  daily  weather  maps.  They  gathered 
observations  from  as  many  stations  as  possible,  and  charted  on  a  single  map 
all  the  facts  recorded  for  a  certain  definite  time,  as  for  noon  on  a  certain  date ; 
then  again  for  midnight; 
for  the  next  noon,  and 
so  on  ;  thus  producing 


what  are  called  synoptic 
maps,  and  gaining  from 
them  a  series  of  views 
of  actual  atmospheric 
processes  over  a  large 
region.  Facts  that  can 
be  with  difficulty  per- 
ceived from  observations 
at  a  single  station  were 
brought  clearly  to  sight 
by  combining  the  re- 
cords of  many  stations ; 
but  while  the  prepara- 
tion of  modern  synoptic 
maps  is  simplified  by 
the  prompt  telegraphic 
concentration  of  numer- 
ous systematic  observa- 
tions made  by  trained 

members  of  a  weather  service,  as  will  be  more  fully  described  in  Chapter  XIII, 
Dove  and  Kedfield  had  a  most  laborious  task  in  searching  out  the  scattered 
records  of  observers  who  followed  no  uniform  plan,  and  who  made  no  regular 
reports  to  any  central  office. 

Rules  based  on  these  discoveries  were  soon  framed,  by  which  the  mariner 
may  generally  avoid  the  more  dangerous  central  hurricane  winds,  and  even 
utilize  the  more  moderate  marginal  gales  to  hasten  on  his  way.  At  first,  the 
surface  winds  of  cyclones  were  thought  to  move  in  circles  around  the  center ; 
but  it  has  since  been  shown  that  their  incurvature  from  a  circular  course  in 


FIG.  53. 


188 


ELEMENTARY    METEOROLOGY. 


the  outer  part  of  the  whirl,  and  particularly  in  the  rear  of  the  storin  area,  may 
amount  to  as  much  as  two  or  three  points  —  23°  to  34°,  —  although  close  about 
the  center  the  movement  of  the  wind  is  much  more  nearly  circular.  The 
isobars  and  vorticular  winds  of  a  violent  hurricane  on  the  coast  of  Florida  at 
Greenwich  noon,  August  22,  1887,  are  illustrated  in  Fig.  53.  The  steamer 
"  Knickerbocker  "  passed  through  the  center  of  this  cyclone  in  the  evening  of 
August  23 ;  in  the  afternoon,  the  wind  blew  a  hurricane  from  the  east,  with 


FIG.  54a. 


FIG.  546. 


&M-^ 


heavy  rain,  the  sea  a  mass  of  foam  ;  at  9  P.M.,  the  wind  suddenly  lulled ;  at 
10.15,  the  wind  suddenly  came  out  of  the  west-northwest  with  fearful  violence, 
terrific  rain,  and  the  sea  was  again  lashed  to  foam  ;  the  next  morning,  the  wind 
moderated  with  rising  barometer,  as  the  vessel  steamed  southward  and  the 
storm  moved  northward. 

The  spiral  character  of  the  winds  is  again  indicated  in  Fig.  54a  and  by 

showing  the  winds  of  a  Cuban  hurricane  on 
September  3  and  5,  1888;  the  increase  in  the 
size  of  the  storm  between  these  two  dates  is 

\..  ;,;£  ..-••        ^         noteworthy.    From  examples  like  these,  a  graphic 
*--\.' '''!''.'''$£%•'.' **''     >'     index>  Fig-  55>  nas  been  prepared  by  our  Hydro- 
"\:- ..V...X-  .' A '•;>''•••        *        graphic  Office,  in  which  the  winds  of  any  given 
^^     .x-'^Tj  •  \    I.     V      direction  are  indicated  in  their  average  position 
^       -w''  A'    '-s  ''.  with  respect  to  the  storm  center;  all  those  from 

the  same  compass  point  being  on  the  same 
dotted  line.  But  while  the  surface  winds  turn 
to  spiral  courses,  the  lower  clouds  follow  nearly 
circular  paths  around  the  storm  center,  and  a 
line  at  risjlit  angles  to  their  movement  leads 
almost  directly  to  the  region  of  greatest  .1  anger.  Knowing  the  average  size  of 
the  cyclonic  area  and  the  signs  by  which  its  coming  is  heralded;  knowing  that 


Pio.  66. 


CYCLONIC    STORMS    AND    WINDS.  189 

the  cyclonic  winds  always  whirl  to  the  left  in  this  hemisphere  and  to  the  right 
in  the  other ;  and  remembering  that  cyclones  move  at  a  moderate  velocity 
westward  and  poleward  while  still  within  the  meteorological  tropics,  and 
slowly  northward  as  they  pass  to  higher  latitudes,  it  is  possible  to  perceive 
their  coming  betimes,  and  to  turn  aside  from  their  dangerous  centers,  thus 
greatly  diminishing  the  dangers  that  they  bring. 

The  essential  elements  of  the  rules  laid  down  for  mariners  are  :  first,  avoid 
running  before  the  wind,  particularly  on  the  right  of  the  track  (left  in  the 
southern  hemisphere),  for  this  may  lead  the  vessel  along  the  incurving  path 
towards  the  storm  center;  second,  lie  to  on  the  starboard  tack  (port  tack  in 
the  southern  hemisphere) ;  that  is,  trim  the  sails  so  as  to  take  the  wind  on  the 
starboard  beam  or  quarter,  for  in  this  way  the  vessel  will  necessarily  sail  away 
from  the  storm  center  into  quieter  weather.  But  it  should  be  added  that  no 
formal  rules  will  replace  the  good  seamanship  that  comes  from  experience,  or 
save  a  vessel  from  the  many  unlooked-for  dangers  of  a  storm. 

The  most  dangerous  portion  of  a  cyclone  lies  somewhat  forward  and  to  the 
right  of  its  center  in  this  hemisphere,  to  the  left  in  the  other  ;  for  here  the 
winds  are  usually  most  violent,  here  the  advancing  center  of  the  storm  may 
overtake  a  vessel  that  is  attempting  to  run  forward  and  cross  its  track,  and 
here  the  curvature  of  the  path  of  the  storm  constantly  brings  the  violent 
center  closer  to  the  ship.  Vessels  taken  unaware  in  this  dangerous  quarter  of 
a  cyclone  may  be  unable  to  escape  its  violence. 

A  modern  addition  to  the  older  rules  for  diminishing  the  danger  of  storms 
at  sea  is  to  spread  oil  on  the  waves,  whereby  their  height  is  lessened  and  they 
break  less  frequently  over  the  vessel.  Even  a  small  quantity  of  oil  allowed  to 
drip  from  a  bag  hung  over  the  vessel  to  windward  has  been  found  by  repeated 
experiment  to  be  of  great  service. 

220.  Evidence  of  convectional  action  in  tropical  cyclones.  In  attempting 
to  explain  these  violent  commotions  of  the  atmosphere,  we  must  resolve  the 
motion  of  the  wind,  AB,  Fig.  56,  into  two  components,  one  of 
which,  AD,  is  directed  radially  inward  toward  the  center,  (7, 
while  the  other,  A  E,  runs  around  the  center  with  increasing 
rapidity  as  the  center  is  approached.  In  the  outer  part  of  the 
cyclone,  the  radial  component  is  almost  equal  to  the  circular ; 
near  the  center,  the  circular  component  is  much  the  greater  of 
the  two.  Indeed,  if  the  circular  component  of  the  wind's 
motion  were  absent,  and  the  air  moved  simply  as  a  radial 
inflow  instead  of  in  a  spiral  whirl,  its  velocity  would  be  so 
moderate  that  it  would  not  reach  the  violence  of  a  storm  wind. 
Different  causes  control  the  two  components  of  the  motion  of 
the  wind  ;  we  shall  first  look  for  the  cause  of  the  radial  inflow.  FIG.  5C. 


190  ELEMENTARY    METEOROLOGY. 

Inasmuch  as  the  central  barometric  pressure  remains  low,  or,  as  the  storm 
grows  stronger,  even  becomes  lower  than  before,  in  spite  of  the  gradual  spiral 
inflow  of  the  winds,  some  escape  for  the  air  from  the  central  region  must  be 
inferred.  An  upward  escape  is  indicated  by  the  occurrence  of  the  great  cloud 
mass  above  and  by  the  drenching  rains  that  fall  from  it ;  for  no  cause  of  an 
extensive  condensation  of  vapor  is  so  effective  as  the  mechanical  cooling  of 
ascending  currents.  The  upward  escape  is  in  turn  confirmed  by  the  outward 
spreading  of  the  cirrus  plumes  in  all  directions  aloft.  There  can  be  little 
doubt  that  the  air,  which  slowly  approaches  the  center  below  as  it  whirls 
around  in  rapid  circuits,  finds  an  escape  by  ascending  gradually  in  the  area 
around  the  central  calm,  whirling  as  it  rises  and  flowing  spirally  outward  far 
above  sea  level.  The  lower  warm  damp  air,  cooling  a  little  by  expansion  as  it 
advances  towards  the  central  lower  pressure,  and  by  the  fall  of  rain  from  the 
clouds  above,  forms  the  lowest  scud  clouds  flying  before  the  wind.  Cooling 
much  more  in  its  spiral  ascent  around  the  center,  it  forms  the  heavy  cloud 
mass  from  which  fall  the  drenching  rains  that  always  accompany  tropical 
cyclones.  The  last  of  the  condensed  vapor,  excluded  at  the  highest  levels  at 
temperatures  below  freezing,  takes  the  form  of  ice  crystals  and  makes  the  long 
outflowing  cirrus  plumes.  Beyond  the  extremity  of  these  clouds,  it  is  probable 
that  a  gradual  descending  motion  of  the  outflowing  air  takes  place  in  the 
pericyclonic  ring  of  higher  pressure  and  clear  dry  air,  by  which  the  cyclone  is 
surrounded. 

The  indications  of  inflow,  ascent,  outflow  and  descent  in  systematic  order 
thus  suggest  that  the  inward  component  of  the  surface  winds  is  one  member 
of  a  large  convectional  circulation,  such  as  was  explained  in  Chapter  VI ; 
while  the  whirling  component  is  produced  in  some  other  way.  If  the 
convectional  origin  of  the  inflowing  surface  winds  is  accepted,  their  movement 
must  be  ascribed  to  the  inferred  settling  down  and  creeping  in  of  heavy 
marginal  air;  for  convection  always  depends  primarily  on  a  descending 
motion  under  the  pull  of  gravity.  But  before  the  action  of  gravity  can 
produce  a  descending  movement  in  the  atmosphere,  there  must  be  an  excess  of 
heat  localized  in  the  area  which  afterwards  becomes  the  storm  center  with  its 
ascending  currents  ;  and  this  seems  to  be  the  fact.  In  all  cases  where  tropical 
cyclones  have  been  traced  backwards  towards. their  source,  they  seem  to  have 
come  from  near  the  warm  equatorial  regions,  where  the  accumulation  of  over- 
warm  and  moist  air  is  a  characteristic  feature.  Yet  if  such  is  their  origin, 
they  should  be  expected  at  all  seasons,  for  the  middle  torrid  zone  has  a 
•  •••ntinual  summer  and  an  ever-ready  instability.  The  following  sections  will 
show,  however,  that  this  is  not  the  case,  and  will  also  suggest  a  sufficient 
reason  for  the  limitation  of  the  occurrence  of  tropical  cyclones  to  particular 
seasons. 


CYCLONIC    STOKMS    AND    WINDS. 


191 


221.  Seasons  and  regions  of  tropical  cyclones.  There  are  five  regions 
in  which  tropical  cyclones  appear  with  characteristic  regularity.  The  first  to 
be  mentioned  is  in  the  North  Atlantic,  where  they  are  thought  to  begin  at  a 
distance  of  six  or  eight  degrees  north  of  the  equator,  and  somewhat  towards 


FIG.  57. 


the  warmer  western  side  of  the  ocean.  The  cyclones  are  small  when  first 
observed,  with  almost  radial  winds,  but  they  increase  in  size  and  violence 
and  rotary  motion  as  they  move  westward,  then  northwestward  over  the  Lesser 
Antilles,  still  recurving  to  the  right  until  they  advance  directly  north  for  a 


ELEMENTARY   METEOROLOGY. 

short  distance  opposite  Florida,  and  then  turn  off  to  tne  northeast  along 
or  near  our  coast,  frequently  continuing  as  far  as  northwestern  Europe  before 
they  fade  away.  It  was  in  the  study  of  these  cyclones  from  1830  to  1850  that 
Redfield  contributed  so  much  to  the  law  of  storms.  Fig.  57  gives  the  tracks 
of  many  Atlantic  hurricanes  traced  by  Redfield  and  Reid,  reduced  from  one  of 
Redfield's  latest  essays,  published  in  1854  ;  but  it  is  probable  that  the  curves 
are  here  too  regularly  drawn.  This  map  forms  the  basis  for  the  indications  of 
storm  tracks  on  the  Pilot  Charts  of  the  North  Atlantic,  issued  monthly 
by  our  Hydrographic  Office  at  Washington. 

A  special  account  has  been  issued  by  the  Hydrographic  Office  of  the  St. 
Thomas-Hatteras  hurricane  of  September  3  to  12,  1889.  It  was  felt  on  the 
Windward  Islands  on  September  2.  On  the  3rd,  it  had  a  diameter  of  about 
500  miles  with  a  calm  center  about  16  miles  across,  and  thus  passed  close  to 
the  north  of  St.  Thomas.  On  the  4th,  it  was  north  of  Puerto  Rico,  where 
northerly,  westerly,  and  southerly  gales  were  successively  felt ;  the  clouds  of 
the  cyclone  were  seen  on  this  day  from  Turk's  Island,  causing  alarm  until  they 
drifted  away  to  the  north.  On  the  5th,  the  storm  center  lay  about  300  miles 
north-northwest  of  Santo  Domingo ;  its  enormous  waves  had  overspread  the 
greater  part  of  the  western  Atlantic  in  a  heavy  swell,  felt  even  at  Nantucket. 
On  the  6th,  it  was  midway  between  Cuba  and  the  Bermudas,  still  having  violent 
winds  and  heavy  clouds  and  rain.  By  the  8th,  it  lay  about  300  miles  east- 
southeast  of  Cape  Hatteras,  and  a  northeast  gale  blew  along  our  coast  from 
Maine  to  Carolina ;  on  this  and  the  subsequent  days,  great  damage  was 
done  by  the  surf  on  the  New  Jersey  coast.  The  storm  moved  slowly  north- 
ward, and  after  the  10th,  when  the  center  was  off  Norfolk,  its  winds  weakened, 
and  on  the  12th  its  fury  was  exhausted.  Hundreds  of  vessels  that  had  been 
storm-bound  in  our  harbors  set  sail  in  the  fair  weather  that  followed.  Fig.  58 
represents  the  great  cyclone  of  November,  1888,  on  the  25th  of  that  month, 
on  its  way  along  our  coast.  Fig.  106  shows  the  isobars  and  gradients  of  the 
western  half  of  the  same  storm  on  November  28. 

The  season  at  which  tropical  cyclones  are  observed  in  the  North  Atlantic 
is  limited  to  the  late  summer  and  early  autumn  ;  the  months  in  which  they 
are  commonest  being  August,  September,  and  October,  while  they  are 
practically  unknown  from  December  to  June.  Their  annual  number  seldom 
exceeds  six  or  eight ;  and  only  a  few  of  these  may  reach  the  greatest  violence. 

In  seeking  a  cause  for  their  coming  to  the  West  Indies  in  the  months 
about  the  autumnal  equinox,  it  is  noticed  that  in  these  months  the  equatorial 
calms  or  doldrums  of  the  Atlantic  migrate  farthest  north  of  the  equator,  and 
that  in  tracing  the  cyclones  backward  along  their  track,  it  is  in  the  calm 
n-jjion  of  warm,  moist  air  between  the  steady  trades  that  the  apparently 
convectional  overturning  of  the  West  Indian  cyclones  has  its  beginning.  Such 
a  region  of  quiet  air  under  strong  sunshine  is  the  natural  seat  of  the  most 


CYCLONIC    STORMS    AND    WINDS.  198 

pronounced  convectional  action.  The  air  being  quiet  becomes  warm  and  well 
moistened  by  evaporation  ;  the  warm  and  moist  air  becomes  unstable  and  takes 
on  a  gradual  convectional  overturning,  and 'from  this  beginning  the  develop- 
ment of  the  cyclone  is  thought  to  proceed. 

In  the  same  way,  the  western  and  warmer  equatorial  Pacific  southeast  of 
Asia  furnishes  to  the  Philippine  and  Japanese  islands  and  the  neighboring 


FIG.  58. 

coast  of  China  a  succession  of  cyclones,  there  called  by  the  Chinese  name  of 
typhoons,  in  the  months  of  August,  September,  and  October  nearly  every  year, 
when  the  Pacific  doldrums  are  farthest  north.  It  was  concerning  one  of  these, 
crossing  the  Philippine  islands  in  1882  on  its  usual  northwestward  course  for 
that  latitude,  and  making  automatic  record  of  its  weather  in  the  meteorological 
observatory  at  Manilla,  that  the  statements  were  made  above  regarding  the 
high  temperature  and  low  humidity  in  the  calm  eye  of  a  cyclone.  Certain 


l',»4  ELEMENTARY    METEOROLOGY. 

cyclones  that  have  been  traced  for  a  relatively  short  distance  along  a  north- 
westward course  in  the  north  torrid  zone  of  the  Pacific  ocean  west  of  Central 
America  are  also  supposed  to  have  originated  in  the  equatorial  calm  belt. 
They  are  not  known  to  cross  the  trade  wind  belt. 

Again,  in  the  southern  Indian  Ocean,  the  islands  of  Mauritius  and  Reunion 
are  annually  visited  by  a  series  of  cyclones,  here  coining  from  the  northeast, 
recurving  in  latitude  25°  or  30°  south,  and  then  passing  off  southeastward  to 
the  south  temperate  regions.  It  was  from  the  study  of  these  that  Meldrum, 
the  meteorologi&t  of  Mauritius,  was  among  the  first  to  announce  the  true 
incurvature  of  cyclonic  winds  from  their  supposed  circular  course.  In  the 
northern  hemisphere,  the  cyclone  season  occurred  when  the  doldrums  stood 
farthest  north  of  the  equator  ;  in  this  southern  ocean,  they  spring  up  when 
the  doldrums  move  to  their  farthest  southern  latitude  ;  that  is,  in  February 
and  March  ;  thus  giving  additional  confirmation  of  their  convectional  origin 
in  the  belt  of  equatorial  calms. 

Cyclones  are  little  known  in  the  South  Pacific  ocean,  but  are  occasionally 
met  with  in  the  region  east  of  Australia,  where  the  ocean  is  warm  and  where 
the  trade  winds  are  weak.  Their  season  of  occurrence  is  in  the  later  summer 
or  early  autumn  of  the  southern  hemisphere.  It  was  in  this  region  that  a 
hurricane  in  March,  1889,  wrecked  or  injured  many  naval  vessels  belonging  to 
different  nations  at  Apia,  Samoa ;  and  that  a  hurricane  in  March,  1886,  was 
severely  felt  on  the  Fiji  islands. 

Cyclones  occur  with  dreaded  violence  in  the  northern  gulfs  of  the  Indian 
Ocean,  particularly  in  the  Bay  of  Bengal.  It  was  in  the  study  of  these  storms 
that  Piddington  first  proposed  the  name  cyclone,  fifty  years  ago  ;  and  here  in 
later  years  the  Meteorological  Service  of  India  has  gathered  the  fullest 
information  now  in  hand  concerning  the  early  stages  of  cyclonic  action.  The 
violent  cyclones  of  the  Bay  of  Bengal  are  unlike  those  of  other  parts  of  the 
world  in  occurring  in  two  seasons  instead  of  in  only  one.  They  occur  in 
the  southern  and  central  parts  of  the  Bay  first  in  April,  May,  and  June,  and 
again  in  October  and  November.  In  the  intervening  summer  months,  less 
violent  cyclonic  storms  are  formed  in  the  north  of  the  Bay  and  on  the  land. 
Their  general  progression  is  to  the  northwest,  but  they  turn  more  to  the  north 
or  even  to  the  northeast  in  the  earlier  and  later  months  of  the  year.  Good 
reason  for  the  double  season  of  occurrence  of  the  more  violent  cyclones  is 
found  in  the  prevalence  of  calms  over  the  Bay  of  Bengal  in  these  two  seasons 
of  the  year  :  first,  when  the  sun  advances  northward,  and  again  when  it  returns 
southward ;  while  the  cyclonic  storms  ofymidsummer  are  formed  farther  north, 
but  often  on  land,  corresponding  to  the  northernmost  position  of  the  hoat 
equator.  This  gives  still  further  Around  for  ascribing  tropical  cyclones  to  the 
calm  areas  of  the  migrating  equatorial  belt. 


CYCLONIC    STORMS    AND    WINDS. 


195 


222.  Tables  of  cyclone  frequency.  The  tropical  cyclones  in  the  four 
chief  regions  of  their  occurrence  have  been  tabulated  by  various  meteorologists 
to  illustrate  their  distribution  through  the  year,  as  appears  in  the  following 
lists.  The  numbers  are  not  strictly  comparable,  for  no  precise  standard  of 
violence  is  yet  adopted  to  determine  whether  a  storm  shall  be  counted  or  not. 
In  most  of  the  examples,  the  storms  have  not  been  traced  backward  along  the 
track  to  their  source. 


LOCALITY.  —  AUTHOR. 

PERIOD. 

>-a 

I 

| 

<J 

! 

••a 

>, 
's 
•-a 

$ 

•< 

I 

i 

i 

1 

YEAR. 

West  Indies. 

Poey     .... 

1493  to  1855   . 

5 

7 

11 

6 

5 

10 

42 

96 

80 

69 

17 

7 

355 

China  Seas. 

Schiick.    .     .     . 

85  years     .     . 

5 

1 

5 

5 

11 

10 

22 

40 

58 

35 

16 

6 

214 

Bay  of  Bengal. 

Blanford   .     .     . 

To  1876     .     . 

2 

0 

2 

9 

21 

10 

3 

4 

6 

31 

18 

9 

115 

Eliot     .... 

1877  to  1891    . 

_ 

- 

- 

- 

10 

17 

27 

24 

28 

18 

21 

7 

152 

Arabian  Sea. 

Chambers  .    .     ., 

To  1881      .     . 

4 

3 

2 

9 

13 

20 

2 

2 

3 

4 

10 

2 

74 

Eliot     .... 

To  1888     .     . 

2 

0 

0 

8 

9 

5 

0 

0 

1 

3 

8 

1 

37 

6'.  Indian  Ocean. 

Meldrum   .     .     . 

1848  to  1891    | 

63 
19 

76 
6 

49 
19 

33 

24 

11 
11 

1 
2 

1 

1 

0 
0 

0 
0 

2 
3 

14 
13 

29 
10 

279 
108 

The  list  of  cyclones  for  the  West  Indies  includes  all  the  violent  storms  of 
that  region  mentioned  by  older  and  newer  authors  down  to  1855.  It  is  highly 
probable  that  a  number  of  storms  noted  in  the  winter  months  do  not  really 
belong  to  the  class  of  tropical  cyclones.  A  tabulation  has  been  made  for  this 
region  in  recent  years  by  Finley. 

The  figures  for  the  Bay  of  Bengal  for  1877  to  1891  include  many  cyclones 
of  moderate  size  and  intensity  that  formed  in  the  north  of  the  Bay  or  over  the 
adjacent  land,  where  they  were  detected  on  the  daily  weather  maps  of  India. 
These  do  not  exhibit  a  double  period  such  as  appears  in  the  earlier  list,  which 
includes  only  the  larger  and  more  violent  cyclones,  whose  origin  was  in  the 
central  or  southern  part  of  the  Bay.  The  late  winter  storms  of  the  Arabian 
sea  included  in  the  list  by  Chambers  probably  originated,  at  least  in  part,  in 
temperate  latitudes  north  of  the  sea,  and  should  not  be  included  with  the 
rest  as  of  tropical  origin. 

The  data  for  the  South  Indian  ocean  have  been  especially  furnished  by  Mr. 
Meldrum  of  Mauritius.  The  first  series  of  figures  includes  those  cyclones 
whose  progression  has  been  well  determined  ;  the  second  series  gives  the 
storms  which  have  not  been  shown  to  change  their  position,  these  being 
generally  of  brief  duration. 


196  ELEMENT  All  V  METEOROLOGY. 

223,  Early  stages  of  cyclonic  action.  In  most  of  the  oceans  the  early 
stages  of  cyclones  have  not  been  fully  observed  ;  but  in  the  Bay  of  Bengal, 
where  cyclones  are  relatively  numerous,  the  logs  of  many  vessels  passing  to 
and  fro  have  been  carefully  examined  for  the  fortnight  before  the  occurrence 
of  storms,  and  thus  the  conditions  of  their  beginning  have  been' well  determined. 
The  most  notable  antecedent  condition  observed  before  the  appearance  of  a 
cyclone  in  the  Bay  and  presumably  occurring  also  in  other  regions  of  cyclone 
growth,  is  the  uniformity  of  pressure  and  the  quietness  of  the  air  over  the  sea. 
There  may  be  light  local  breezes,  but  there  is  no  persistent  movement  of  the 
atmosphere  such  as  prevails  during  the  occurrence  of  the  summer  or  winter 
monsoon,  when  the  winds  sweep  steadily  across  the  whole  breadth  of  the 
waters.  The  quiet  air  becomes  over-warm  and  moist,  and  clouds  hide  the 
sky  in  the  calm  region  ;  with  this,  the  pressure  decreases  slightly,  and  gentle 
marginal  breezes  are  established  towards  and  around  the  central  district.  Rain 
sets  in  under  the.  central  clouds,  the  barometer  falls  to  lower  readings,  the 
winds  blow  stronger  and  with  more  definite  courses,  all  conspiring  to  form  a 
vorticular  whirl  about  the  center  of  lowest  pressure.  It  is  probable  that  the 
irregularity  sometimes  noticed  in  the  winds  at  the  inception  of  the  cyclone 
results  from  the  imperfect  development  of  several  low-pressure  centers  ;  but  at 
a  little  later  stage,  one  of  these  alone  survives  and  becomes  the  eye  of  the 
cyclone,  and  the  isobars  assume  an  almost  circular  form  around  it.  As  the 
pressure  falls  towards  its  lowest  value  and  the  winds  attain  their  greatest. 
strength,  the  center  of  the  cyclone  advances  from  its  first  vague  position  along 
the  usual  northwest  course.  It  progresses  with  growing  violence  until  it 
reaches  the  land  ;  then  its  on-shore  winds  sweep  the  waters  of  the  Bay  <>v«-r 
the  low  delta  plain  of  eastern  Bengal,  where  the  inhabitants  have  thus  been 
drowned  by  the  tens  and  almost  by  the  hundreds  of  thousands.  Further 
inland,  the  storm  generally  weakens  ;  and  on  approaching  the  mountains  to 
the  north  or  the  elevated  plateau  country  in  the  south  of  the  peninsula,  it 
fades  away.  The  early  stages  of  storm  growth  are  here  so  well  observed  and 
indicate  so  clearly  a  convectional  beginning  for  the  cyclone  that  this  theory 
of  their  origin  here  and  in  other  parts  of  the  torrid  zone  is  now  generally 
accepted. 

While  the  foregoing  paragraphs  give  good  reason  for  associating  the 
formation  of  cyclones  with  convectional  action  in  the  equatorial  calms  or  in 
tin-  weaker  part  of  the  trade  winds,  no  full  explanation  has  yet  been  givm  f<>r 
their  limitation  to  special  seasons  of  occurrence,  when  the  calms  are  furthest 
from  the  equator.  The  conditions  for  convectional  overturning  an-  alwa\s 
present  in  the  equatorial  calm  belt  in  a  greater  or  less  degree;  the  air  there 
loiters  about,  moving  gently  in  li^ht  baffling  breezes,  reaching  as  lii^li  a 
temperature  as  is  found  anywhere  over  the  ocean,  always  well  moistened  by 
evaporation  from  the  sea,  and  ready  to  ascend  whenever  cooler,  heavier  air 


CYCLONIC    STORMS    AND    WINDS.  197 

flows  in  beneath  it.  Some  additional  reason  besides  instability  must  therefore 
be  found  to  limit  the  development  of  convectional  cyclones  to  that  season  when 
the  calms  migrate  on  the  sea  surface  farthest  from  the  equator,  either  into  the 
northern  or  southern  hemisphere.  The  reason  sought  for  may  be  found  when 
it  is  remembered  that  a  direct  convectional  indraft  of  the  surface  winds  cannot 
alone  produce  any  whirling  motion,  and  that  the  terrific  blasts  of  the  wind 
near  the  center  of  tropical  cyclones  are  always  in  an  almost  circular  path. 
Cyclones  must  therefore  be  essentially  vorticular  storms,  and  although  begun 
by  convectional  action,  some  supplementary  cause  must  set  them  in  rotation. 

224.  Effect  of  the  earth's  rotation.  Tropical  cyclones  being  essentially 
vorticular  storms,  a  sufficient  explanation  of  their  occurrence  only  when  their 
place  of  origin  is  removed  from  the  equator  must  have  already  come  to  mind 
from  the  account  of  the  deflecting  action  of  the  earth's  rotation  given  in 
Chapter  VI.  Instability  and  convection  occur  in  the  doldrums  at  all  seasons, 
as  shown  by  the  numerous  thunder  storms  and  heavy  rains  of  the  calm 
belt,  but  the  establishment  of  a  convectional  whirl  with  a  definite  direction 
of  rotation  can  take  place  only  when  the  doldrums  are  far  enough  from  the 
equator  to  give  the  deflecting  force  an  effective  value  and  allow  it  to  require 
the  winds  to  depart  systematically  from  directly  radial  lines  of  inflow.  The 
departure  of  all  the  inflowing  winds  being  to  the  right  in  this  hemisphere,  or 
to  the  left  in  the  other,  they  must  all  conspire  to  produce  a  left-handed  whirl 
if  they  begin  north  of  the  equator,  or  a  right-handed  whirl  if  they  are  developed 
south  of  the  equator.  The  origin  of  the  circumferential  component  of  the 
wind's  motion,  AE,  Fig.  56,  is  thus  accounted  for. 

Recalling  the  explanations  of  Section  133,  it  will  be  understood  that  any 
mass  of  calm  air  in  the  doldrums  or  elsewhere  may  be  compared  to  a  paper 
disc  attached  to  an  artificial  globe  ;  or  to  a  vessel  of  water  on  a  turning  table  ; 
and  hence  that  while  its  parts  are  quiet  with  respect  to  the  surface  of  the  sea 
on  which  they  lie,  they  nevertheless  possess  a  movement  of  rotation  with 
respect  to  their  center  in  consequence  of  their  residence  on  a  rotating  planet. 
The  air  at  the  equator  corresponds  to  a  stand-still  of  the  turning  table.  The 
air  in  the  calms  north  of  the  equator  corresponds  to  the  water  in  the  vessel 
when  the  table  is  turning  slowly  from  right  to  left ;  south  of  the  equator, 
when  the  table  is  turning  from  left  to  right.  The  air  in  temperate  latitudes 
corresponds  to  a  faster  rotation  of  the  table.  If  these  relations  are  appreciated, 
there  can  be  no  difficulty  in  explaining  the  limitation  of  tropical  cyclones  to 
certain  seasons,  when  the  calms  in  which  they  begin  have  migrated  far 
enough  north  or  south  of  the  equator. 

Recalling  next  the  experiments  with  the  eddies  of  water  in  the  rotating 
vessel  (Sect.  135),  the  violence  of  the  cyclonic  whirl  may  be  appreciated  ; 
it  beins^  understood  that  while  the  water  eddy  is  discharged  downward,  the 


198  ELEMENTARY   METEOROLOGY. 

atmospheric  eddy  is  discharged  upward.  It  has  been  seen  that  if  the  water  is 
discharged  after  it  has  been  given  a  gentle  rotation,  the  outflowing  currents 
soon  develop  a  violent  central  vortex,  where  the  velocity  of  the  threads  of  water 
is  much  greater  than  the  velocity  of  rotation  at  the  margin  of  the  vessel,  and 
very  much  greater  than  was  seen  at  any  part  of  the  discharge  when  there  was 
no  rotation.  The  centrifugal  force  developed  by  the  rapid  whirling  of  the 
water  on  a  small  radius  produces  a  distinct  depression  of  the  water  surface  at 
the  center ;  the  whirl  may  become  so  violent  as  to  form  an  empty  core  as  a 
result  of  the  excess  of  the  horizontal  centrifugal  force  in  the  whirl  over  the 
downward  action  of  gravity.  Finally,  the  discharge  of  rotating  water  requires 
more  time  than  the  discharge  of  quiet  water ;  from  having  taken  a  quarter  of 
a  minute  when  quiet,  it  may  occupy  forty  or  fifty  seconds  when  rotating. 

These  simple  experiments  illustrate  motions  that  are  analogous  to  the 
flowing  of  the  surface  winds  in  the  convectional  overturnings  of  the  equatorial 
calms,  with  the  respective  parts  inverted.  The  top  of  the  water  corresponds 
to  the  bottom  of  the  atmosphere.  The  downward  discharge  of  the  water  from 
the  vessel  corresponds  to  the  convectional  ascent  of  the  air  in  the  doldrums. 
The  direct  discharge  of  the  quiet  water  illustrates  the  simple  convectional 
overturning  of  the  air,  when  the  calms  are  near  the  equator  and  no  cyclonic; 
whirls  are  produced.  The  whirling  escape  of  the  rotating  water  represents  the 
whirling  inflow  of  the  winds  when  the  calms  stand  far  enough  from  the  equator 
for  their  breezes  to  be  governed  by  the  deflective  action  of  the  earth's  rotation. 
Moving  gently  inward  at  first,  the  whirling  velocity  continually  increases  as 
the  center  is  approached,  and  the  wind  attains  a  full  hurricane  violence  close 
around  the  area  of  the  central  calm,  where  it  follows  an  almost  circular  path. 

The  origin  of  tropical  cyclones  thus  appears  to  be  well  worked  out.  Being 
of  convectional  nature,  they  are  not  formed  in  the  steadily-moving  trades,  but 
only  when  the  trades  weaken  in  the  loitering  doldrums,  where  the  lower  air 
becomes  excessively  warm  and  moist ;  being  essentially  whirling  storms,  they 
cannot  develop  when  the  doldrums  are  close  to  the  equator,  where  the  inflowing 
currents  are  not  required  to  unite  in  forming  a  systematic  vortex  ;  but  only 
when  convectional  action  begins  at  some  distance  north  or  south  of  the 
equator.  The  definite  and  rational  association  of  these  various  conditions  of 
storm  growth  gives  warrant  for  much  confidence  in  the  convectional  theory  of 
the  formation  of  tropical  cyclones. 

225.  Absence  of  tropical  cyclones  from  the  South  Atlantic.  All  the  torrid 
oceans  are  visited  by  tropical  cyclones,  except  the  South  Atlantic.  They  are 
not  common  in  the  broad  South  Pacific ;  but  they  have  been  observed  there  and 
with  terrible  violence,  as  at  Samoa  in  1889.  But  the  south  torrid  zone  of  thn 
Atlantic  has  no  record  of  true  cyclones.  The  reason  for  this  peculiar  exception 
is  apparent  when  the  attitude  of  the  doldrums  in  January  and  February  is 


CYCLONIC    STORMS    AND    WINDS. 


199 


examined  in  Fig.  59.     The  shaded  areas  in  this  figure  represent  the  location 

of  the  equatorial  rain  belt  for  the  northern  and  southern  positions  of  the  heat 

equator.      The    trades 

die  out  as  they  enter 

this  belt,  their  limits 

being   indicated  by 

broken  or  dotted  lines, 


.  _  TR5.PJC.  OF_CANCER_  .  _  ( 


AFRICA 


SOU 


A   M    E 


TROPIC  OF  CAPRICORN 


-30° 


FIG.  59. 


and  the  calms  of  the 
doldrums  are  found 
between  these  limits. 
The  calms  migrate 
abouH;  ten  degrees 
northward  in  our  late 
summer,  and  there  give 
forth  cyclones  that 
travel  off  toward  the 
West  Indies  ;  but  they 
never  migrate  so  far 
south  of  the  equator, 
being  held  back  by  the 
great  current  of  rela- 
tively cool  water  that 
advances  from  the 
Antarctic  ocean  along 

the  west  coast  of  Africa,  as  has  already  been  dwelt  upon  in  describing  the 
distribution  of  temperature  (Sect.  82).  The  South  Atlantic  is  therefore  the 
only  ocean  into  which  the  doldrums  do  not  migrate.  It  is  also  the  only  ocean 
not  visited  by  tropical  cyclones. 

226.  Latent  heat  from  rainfall.  When  the  cloud  mass  of  a  cyclone  is 
formed  and  its  rainfall  has  begun,  then  not  only  the  sensible  heat  of  the  warm 
air  but  the  latent  heat  of  the  condensed  vapor  promotes  the  convectional  action 
of  the  storm.  The  consideration  of  this  double  process  has  already  been  given 
in  the  chapter  on  clouds  :  here  one  of  its  most  important  applications  is 
encountered.  As  the  warm  air  ascends  in  its  spiral  whirl  around  the  central 
region  of  a  cyclone,  it  must  expand  ;  and  at  first  it  draws  only  on  its  sensible 
heat  for  the  energy  needed  to  push  away  the  surrounding  air.  Its  temperature 
then  falls  at  the  rapid  rate  of  1°.6  for  every  300  feet  of  ascent ;  but  as  the 
whole  mass  is  very  damp,  a  moderate  ascent  is  sufficient  to  reduce  the 
temperature  sufficiently  to  overtake  the  falling  dew-point  and  cause  condensa- 
tion. At  this  moderate  altitude  the  clouds  begin  to  form.  Expansion 
continues  during  ascent  to  greater  heights,  but  the  energy  for  expansion  is 


200  ELEMENTARY    METEOROLOGY. 

then  drawn  from  two  sources  ;  a  part  comes  from  the  sensible  heat  of  the 
a.-M-eiiding  air,  and  the  remainder  from  the  latent  heat  of  the  vapor  that  is 
condensed  in  the  ascent. 

Here,  as  in  all  cases  where  vapor  is  condensed,  it  may  be  looked  on  as 
giving  up  the  store  of  energy  that  was  acquired  from  absorbed  insolation  when 
the  vapor  was  formed,  perhaps  many  days  before  and  hundreds  of  miles  away. 
In  the  case  of  tropical  cyclones,  the  store  is  abundant  and  its  aid  is  most 
effective.  The  cyclonic  inflow  comes  at  first  from  the  doldrums,  and  after- 
wards from  the  trades  when  the  storm  area  increases  and  when  its  progression 
carries  it  to  higher  latitudes.  The  air  thus  supplied  lias  a  high  temperature 
and  a  high  relative  humidity.  Enormous  masses  of  heavy  clouds  are  formed, 
and  rain  falls  from  them  in  drenching  torrents.  The  diagram  in  Section  197 
has  shown  that  it  is  precisely  under  these  conditions  that  the  liberation  of 
latent  heat  causes  the  greatest  retardation  of  cooling  in  an  ascending  current, 
and  that  a  convectional  ascent  may  reach  its  greatest  height.  The  greater  the 
altitude  of  the  cyclonic  mass  in  which  the  temperature  is  higher  than  that  of 
the  surrounding  air,  and  the  greater  the  excess  of  the  central  temperature,  the 
stronger  the  gradients  and  the  more  violent  the  winds.  As  both  tl>e  excess  of 
central  temperature  and  the  altitude  of  ascent  are  greatly  promoted  at  high 
temperatures  by  the  condensation  of  v,apor  and  the  liberation  of  its  latent  heat, 
it  is  manifest  that  the  presence  of  vapor  is  a  very  important  element  in  the 
development  of  tropical  cyclones. 

In  order  to  realize  the  enormous  amount  of  energy  needed  to  develop  a 
tropical  cyclone,  we  may  quote  a  comparison  that  lias  been  drawn  between 
such  a  storm  and  a  large  ocean  steamer.  The  air  in  a  cyclone  100  miles  in 
diameter  and  a  mile  high  weighs  as  much  as  half  a  million  6000-ton  ships  ; 
and  yet  this  enormous  mass  is  set  in  rapid  motion,  averaging  over  40  miles  an 
hour,  in  the  course  of  a  few  days,  and  its  motion  may  be  continued  for  a  week 
or  more.  Again,  the  Cuban  hurricane  of  October  5-7,  1844,  is  calculated  on 
very  moderate  estimates  to  have  worked  during  the  three  days  of  its  progress 
along  our  southern  coast  with  an  energy  of  at  least  473  million  horse-power. 
The  continued  maintenance  of  so  enormously  powerful  a  disturbance  calls  for 
the  rapid  supply  of  a  vast  amount  of  energy  ;  just  as  the  active  steaming  of  a 
large  engine  calls  for  a  plentiful  supply  of  coal  under  its  boilers.  In  the  case 
of  a  fully-developed  tropical  cyclone,  it  is  believed  that  the  energy  is  chiefly 
supplied  from  the  latent  heat  of  the  heavy  rainfall ;  and  reasonable  estimates 
of  the  amount  of  condensation  within  the  storm  disc  show  that  this  source  of 
energy  is  ample  in  amount. 

227.  Occurrence  of  tropical  cyclones  chiefly  over  the  oceans.  The  torrid 
lands  of  Africa  and  South  America  are  not,  as  far  as  observation  ^ors.  visited 
by  tropical  cyclones.  The  insular  and  peninsular  lands  of  southern  Asia  are 


CYCLONIC    STORMS   AND    WINDS.  201 

reached  by  cyclones  that  come  from  the  adjacent  seas,  but  after  leaving  the 
ocean,  their  violence  is  greatly  reduced,  and  if  they  encounter  high  ground 
they  are  broken  up.  It  is  argued  from  this  that  the  development  of  cyclones 
in  the  calms  of  the  doldrums  is  limited  to  those  parts  of  the  belt  which  are 
most  highly  charged  with  vapor,  that  is,  to  the  parts  over  the  oceans  ;  and 
further  that  the  maintenance  of  the  storms  is  difficult  on  the  land  where  the 
water  vapor  is  in  smaller  amount,  and  where  the  vorticular  circulation  of  the 
lower  winds  is  more  or  less  interfered  with  by  mountains  or  plateaus. 

228.    Comparison    of    tropical    cyclones    and    desert  whirlwinds.      An 

instructive  comparison  may  now  be  drawn  between  the  great  tropical  cyclones 
and  the  small  dusty  whirlwinds  of  desert  plains.  Desert  whirlwinds  are 
slender  columns  of  immediate  and  local  formation  and  of  brief  action. 
Tropical  cyclones  are  broad  discs  of  gradual  and  widespread  formation  and 
of  long  endurance ;  their  winds  and  the  vapor  which  is  condensed  in  their 
clouds  may  be  drawn  in  from  districts  several  hundred  miles  away  from  the 
violent  whirl  that  is  generated  around  their  center ;  they  may  last  for  a  week 
or  two,  blowing  night  and  day  and  travelling  thousands  of  miles  away  from 
their  starting  point.  The  desert  whirls  spring  up  about  ten  or  eleven  o'clock 
in  the  morning,  after  the  steepening  rays  of  the  sun  have  warmed  the  barren 
ground  and  the  lower  air  has  been  warmed  from  the  ground  by  conduction 
and  radiation.  Some  little  inequality  of  the  surface  or  a  slight  movement 
received  from  the  winds  aloft  causes  an  upsetting  of  this  unstable  arrangement 
of  the  lower  air,  and  an  inflow  is  thus  begun  towards  the  place  of  ascent ;  but 
as  the  various  inflowing  currents  move  for  too  short  a  distance  to  be  systemat- 
ically influenced  by  the  earth's  rotation,  and  as  their  irregular  flow  does  not 
allow  them  to  meet  precisely  at  a  center,  they  turn  a  little  to  one  side  or  the 
other  according  as  the  stronger  inflow  decides,  and  a  little  whirl  is  then 
developed,  rotating  indifferently  one  way  or  the  other.  As  its  violence 
increases,  dust  and  sand  are  gathered  up  by  the  wind  and  the  lofty,  slender 
column  becomes  visible.  It  may  be  followed  to  a  height  of  several  hundred 
or  even  a  thousand  feet,  where  it  spreads  out  laterally,  the  coarser  sand  soon 
settling  down,  while  the  finer  dust  is  borne  many  miles  away.  The  supply  of 
warm  surface  air  is  soon  exhausted  and  the  whirl  quickly  disappears  ;  but 
within  half  an  hour  another  layer  of  surface  air  may  be  superheated  and  a 
second  whirlwind  arise  from  it.  So  brief  a  process  as  this  is  only  a  slight 
exaggeration  of  the  invisible  convectional  movements  on  which  the  diurnal 
increase  of  the  general  winds  over  the  land  depends. 

The  formation  of  tropical  cyclones  is  much  more  deliberate.  Day  after 
day  the  air  lying  quiet  over  the  sea  becomes  warmer  and  warmer,  the  vapor 
gradually  rises  by  diffusion  and  by  local  convection  to  higher  and  higher 
levels,  nearly  saturating  a  large  volume  of  warm  air.  When  at  last  some 


202  KLEMENTAKY    METEOROLOGY. 

overflow  of  the  expanded  air  is  developed  aloft,  the  more  general  ascent  and 
overclouding  begins  and  a  creeping  in  of  the  surrounding  air  is  established ; 
but  all  these  changes  take  place  very  gradually.  The  observer  only  notes  a 
slow  increase  of  cloudiness  and  a  strengthening  of  the  wind  as  the  days  pass. 
While  the  desert  whirl  quickly  spends  itself,  the  tropical  cyclone  draws  upon 
so  large  a  volume  of  air  that  it  lasts  many  days ;  and  the  prompt  convection 
that  would  drain  away  the  supply,  if  the  inflow  were  directly  radial,  is  greatly 
delayed  by  the  systematic  deflection  of  the  winds  to  the  right  or  left  of  their 
radial  path  and  the  consequent  development  of  a  gigantic  whirl  around  the 
central  area  of  low  pressure.  While  the  desert  whirl  can  continue  only  during 
the  supply  of  warm  air  in  the  hot  hours  of  the  day,  the  cyclone  may  persist 
with  undiminished  energy  over  night,  not  only  because  the  temperature  of  its 
broad  and  thick  layer  of  cloudy  air  is  about  the  same  day  and  night,  but  also 
because  the  greater  supply  of  energy  from  the  latent  heat  of  its  condensing 
vapor  is  furnished  at  night  as  well  as  in  the  day-time. 

While  thus  contrasted  in  many  particulars,  these  two  classes  of  whirls  are 
alike  in  one  essential  feature.  The  winds  of  both  are  driven  by  the  gravitative 
or  downward  pressure  of  a  surrounding  mass  of  heavier  air,  which  settles 
down  as  the  whirling  lighter  air  ascends ;  and  the  essential  cause  of  the 
difference  in  weight  of  the  central  and  surrounding  masses  is  found  in  the 
higher  temperature  of  the  former,  dependent  in  some  way  on  a  greater 
absorption  of  insolation.  Here,  as  in  all  classes  of  winds,  except  those 
expressly  excluded  in  Sections  139  and  140,  the  movement  of  the  atmosphere 
depends  on  the  interaction  of  solar  energy  and  terrestrial  gravitation. 

229,  The  eye  of  the  storm.  Mention  has  already  been  made  of  the  calm 
area  within  the  whirling  hurricane  winds,  where  the  pressure  is  lowest  and 
where  the  rain  ceases  and  the  clouds  sometimes  break  away,  revealing  the 
clear  sky  overhead.  All  these  features  of  the  center  of  a  strong  cyclone  seem 
to  be  easily  explained  as  consequences  of  its  convectional  whirling.  As  the 
winds  are  pushed  in  towards  the  central  area  of  low  pressure,  their  velocity  of 
rotation  around  the  center  becomes  excessive  and  the  centrifugal  force l 
increases  at  a  very  rapid  rate,  as  explained  in  Section  !&">.  The  low  pressure 
at  first  caused  by  high  temperature  is  thus  greatly  intensified  and  the  gradients 
become  very  steep  near  the  center.  The  central  pressure  sometimes  falls  even 
three  inches  below  the  normal  value  of  the  region ;  and  it  is  plain  that  as  the 
winds  approach  so  rarefied  a  region,  they  must  expand  ;IM<!  cool  and  become 
cloudy  ;  it  is  presumably  in  great  part  for  this  reason  that  the  flying  clouds 
around  the  eye  of  a  cyclone  hang  so  low  over  the  sea. 

1  Distinction  should  be  made  between  the  true  centrifugal  force,  arising  from  the  whirling 
of  the  wind  around  the  storm  center,  and  the  deflecting  force  arising  from  the  movement  of 
the  wind  on  a  rotating  earth ;  hut  as  the  former  is  much  the  greater  of  the  two  in  tropical 
cyciones,  it  alone  is  named  in  the  text. 


CYCLONIC    STORMS    AND    WINDS.  203 

The  air  that  is  held  away  from  the  storm  center  by  the  excessive  centrif- 
ugal force  there  developed  aids  the  convectional  overflow  aloft  in  forming  the 
ring  of  slightly  higher  pressure  around  the  storm,  already  referred  to  as  the 
pericyclonic  ring  in  Section  218  :  here  the  clearness  of  the  sky  and  the  fresh- 
ness of  the  air  confirm  the  theoretical  suggestion  that  there  is  a  gentle  descent 
from  aloft ;  and  this  is  further  demonstrated  by  the  relative  calmness  of  the 
air  in  the  ring  of  high  pressure,  and  by  the  slow  outward  spiral  movement 
of  the  air  outside  of  the  ring,  perceptible  in  the  charts  of  the  best-studied 
tropical  cyclones. 

The  inflowing  winds  take  an  almost  circular  course  near  the  center.  They 
are  sometimes  intensified  by  brief  and  violent  gusts  of  terrific  strength  ;  and 
sometimes  deflected  from  their  average  course  by  the  passage  of  subordinate 
eddies  ;  but  on  the  whole,  the  observations  reported  from  vessels  that  happen 
to  be  near  the  vortex  of  a  cyclone  agree  remarkably  well  in  indicating  a 
systematic  whirl  of  the  winds  around  a  single  center.  When  blowing  in  this 
way,  nearly  all  the  strong  centripetal  force  on  the  steep  gradients  is  used  in 
overcoming  the  strong  centrifugal  force  of  the  violent  wind  on  its  small  radius, 
and  there  remains  a  forward  component  of  moderate  value,  sufficient  only 
to  overcome  the  small  resistances  encountered  in  wave-making  and  internal 
friction.  At  moderate  altitudes,  where  the  resistances  are  smaller  than  at 
sea-level,  the  wind  takes  a  more  nearly  circular  course ;  hence  the  movement 
of  the  lower  clouds  is  prevailingly  a  point  or  two  to  the  right  (in  the 
northern  hemisphere)  of  the  surface  wind.  On  land  the  resistances  are  still 
greater,  and  there  the  storm  winds  are  more  nearly  radial  than  at  sea. 

The  approach  of  the  winds  to  the  center  of  a  cyclone  at  sea  is  delayed  by 
their  having  to  follow  a  spiral  course ;  at  the  level  of  the  low-hanging  clouds 
the  winds  are  essentially  circular,  and  cease  further  inflow  at  a  distance  of  ten 
or  fifteen  miles  from  the  center  of  their  whirl.  In  the  meantime  the  convec- 
tional ascent  of  the  air  around  the  central  region  is  continued ;  and  before  any 
part  of  the  indraft  can  enter  very  close  to  the  center  of  the  whirl,  it  is  carried 
upward  to  high  levels,  and  then  turned  spirally  outward  at  the  altitude  of  the 
upper  clouds.  A  certain  space  about  the  center,  commonly  measuring  ten  or 
twenty  miles  in  diameter  and  three  to  five  miles' high,  is  therefore  inaccessible 
to  violent  winds  ;  its  air  is  comparatively  calm,  although  surrounded  by  winds 
of  hurricane  strength.  The  empty  eddy  in  the  center  of  a  vortex  of  water 
and  the  clear  core  often  observed  in  a  dusty  whirlwind  are  analogous  to  the 
calm  central  area  of  a  cyclone :  the  centrifugal  force  of  the  surrounding 
currents  is  so  great  that  they  cannot  approach  closer  to  the  center  before  they 
are  carried  away ;  upward  in  the  air,  downward  in  the  water. 

The  clearness  of  the  eye  of  the  storm  has  been  thus  explained:  The 
hurricane  winds  that  surround  the  central  calm  area  must  impart  some  of  their 
motion  to  the  enclosed  air  by  friction  ;  the  air  thus  given  a  rotary  movement 


J"4  ELEMKNTAKY    METEOROLOGY. 

must  tliereby  acquire  a  certain  centrifugal  force,  and  thus  increase  its  pressure 
a  gainst  and  its  movement  with  the  surrounding  hurricane.  Some  air  will 
thus  be  withdrawn  from  the  calm  center,  and  carried  up  in  the  surrounding 
convectional  whirl.  To  supply  the  air  thus  withdrawn  from  the  margin  of 
the  calm  center,  the  quieter  air  within  must  expand  laterally,  and  thus  allow 
some  air  from  aloft  to  settle  down  to  the  sea,  as  indicated  in  Fig.  60  ;  further- 


FIG.  60. 

more,  the  depression  of  the  isobaric  surfaces  close  around  the  center  is  so 
great  that  an  inflow  may  be  developed  on  them  in  the  higher  atmosphere, 
above  the  level  of  the  whirling  storm.  If  a  slowly  descending  current  can  be 
developed  in  this  way,  it  is  natural  enough  that  it  should  be  clear,  because  it 
will  be  warmed  by  compression,  and  any  clouds  that  may  wander  into  it  will 
soon  be  dissolved.  For  the  same  reason,  if  the  air  descends  actively  even  to 
sea-level,  it  should  be  hot  and  dry,  and  in  strong  contrast  to  the  damp  and 
somewhat  cooled  air  of  the  surrounding  stormy  winds.  This  is  not  always 
the  case,  although  it  was  so  to  a  marked  degree  in  the  hurricane  of  1882  at 
Manilla  on  the  Philippine  islands,  during  the  passage  of  the  central  calm. 

It  should  be  noted  that  while  the  calmness  of  the  central  area  may  be  fully 
explained  on  mechanical  principles  that  certainly  have  application  in  a 
whirling  storm,  the  descent  of  the  air  in  the  central  space  is  merely  a 
suggestion,  plausible  in  certain  respects  and  capable  of  explaining  the 
phenomena  for  whose  explanation  it  is  proposed;  but  not  yet  fully  verified  by 
observation.  Observations  of  the  eye  of  cyclones  while  they  are  passing  over 
islands  may  in  the  future  serve  to  decide  this  question.1 

230,  Comparison  of  tropical  cyclones  and  the  circumpolar  whirl  of  the 
planetary  circulation.  The  comparison  that  may  be  here  drawn  aids  greatly 
in  the  understanding  of  both  systems  of  winds. 

1°.  In  the  circumpolar  whirl  the  center  is  cold,  and  the  air  there  descends  ; 
the  high  polar  pressures  expected  from  low  polar  temperatures  are  reduced  to 
low  pressures  by  the  excessive  centrifugal  force  of  the  whirling  winds ;  and 
the  expected  gradients  towards  the  equator  in  the  lower  air  are  reversed  to 
pole ward  gradients,  except  in  the  trade-wind  belts.  Tn  tropical  cyclones  tin- 
central  region  is  warm,  and  the  air  there  ascends  ;  the  low  pressure  due  to 
liitjh  temperature  is  reduced  to  even  lower  pressure  by  the  centrifugal  force  of 

1  A  full  account  of  the  features  of  the  "eye  of  the  storm"  is  given  by  S.  M.  Ballon  in 
the  American  Meteorological  Journal,  June.  July.  1892. 


CYCLONIC    STOKMS    AM)    AVINDS. 

the  revolving  hurricane ;  and  it  must  be  surmised  that  the  outward  gradients 
which  a  simple  convectional  circulation  would  demand  in  the  upper  air,  are 
thus  reversed  to  inward  gradients. 

2°.  The  central  low  pressures  thus  determined  in  both  systems  of  whirling 
winds  are  accompanied  by  a  partial  re-arrangement  of  the  surrounding 
pressures,  producing  a  ring  of  high  pressures  where  the  air  slowly  descends, 
and  on  whose  circumference  of  no  gradients  there  is  a  belt  of  calms  and  fair 
weather  separating  the  interior  inflowing  spiral  winds  from  a  set  of  exterior 
outflowing  spiral  winds.  The  high-pressure  ring  is  seen  in  the  tropical  belt 
of  high  pressures  of  the  planetary  winds,  outside  of  which  the  trade  winds 
blow  in  an  outward  spiral ;  and  in  the  pericyclonic  ring  of  high  pressure  in 
tropical  cyclones,  beyond  which  are  faint  external  outflowing  winds. 

3°.  Land  masses  with  plateaus  and  mountain  ranges  interfere  with  the 
best  development  of  these  systems  of  winds  and  pressures.  When  tropical 
cyclones  run  ashore,  they  weaken  and  often  disappear.  Similarly,  the 
northern  half  of  the  planetary  circulation,  flowing  over  the  broadest  conti- 
nents, the  highest  plateaus  and  the  most  numerous  mountains,  is  imperfectly 
developed  in  comparison  with  the  southern  half,  where  the  sea  surface  is  so 
little  interrupted. 

4°.  The  return  of  the  planetary  winds  from  the  polar  regions  is  effected 
against  the  (apparent)  gradient  by  virtue  of  the  excessive  centrifugal  force 
previously  developed  while  the  winds  were  obliquely  approaching  the  pole  on 
the  steep  gradients  aloft.  The  observed  outflow  of  the  cirrus  clouds  in  the 
upper  part  of  a  cyclone  may  similarly  have  to  be  performed  against  the  inward 
gradient  of  the  upper  isobaric  surfaces  by  means  of  the  great  centrifugal 
force  that  the  hurricane  has  previously  acquired  as  it  approached  the  storm 
center  on  the  even  steeper  isobaric  surfaces  of  lower  levels. 

5°.  The  observed  calmness  of  the  air  in  the  eye  of  a  tropical  cyclone  lends 
support  to  the  inferred  calmness  of  the  air  in  the  polar  regions  of  the 
planetary  circulation,  mentioned  in  Sect.  142. 

6°.  The  surface  members  of  the  planetary  circulation,  being  reduced  in 
velocity  by  friction  with  the  earth,  are  unable  to  return  to  the  equator  against 
the  poleward  gradients,  and  hence  sidle  obliquely  towards  the  poles.  The 
surface  members  of  the  cyclonic  whirl,  being  retarded  by  friction  and  wave- 
making  to  a  lower  velocity  than  that  gained  by  the  winds  at  the  level  of  the 
clouds,  are  constrained  to  take  a  somewhat  greater  inclination  towards  the 
central  region,  where  the  pressures  are  so  greatly  reduced  by  the  violent 
whirling  of  the  whole  mass. 

It  is  manifest  that  comparisons  of  this  sort  are  of  a  different  order  from 
those  which  are  concerned  with  statistical  values,  based  directly  on  observation, 
such  as  are  commonly  employed  in  climatic  studies.  Each  style  of  comparison 
has  a  value  of  its  own ;  both  must  be  employed  if  the  student  would  reach  an 


200  ELEMENT  AUV    METEOROLOGY. 

appreciation  of  the  science  as  well  as  a  knowledge  of  the  facts  of  meteorology 
Statistical  comparisons  have  been  longer  employed,  and  for  a  time  they 
formed  the  chief  subjects  of  meteorological  study.  Comparisons  of  similar 
phenomena  are  less  usual,  but  not  less  important.  The  one  here  introduced 
was  first  made  by  Ferrel,  to  whom  the  modern  understanding  of  the  theory  of 
the  winds  is  so  largely  due.  It  should  be  carefully  studied,  for  it  is  as 
important  in  meteorology  to  perceive  the  homology  that  exists  between  the 
larger  and  smaller  atmospheric  whirls  as  it  is  in  astronomy  to  understand  that 
the  movement  of  planets  around  the  sun  is  controlled  by  a  system  of  forces 
corresponding  to  that  which  directs  the  movement  of  moons  around  planets. 

231.  The  convectional  theory  of  tropical  cyclones.  The  evidence  detailed 
on  the  preceding  pages  points  very  directly  to  the  conclusion  that  tropical 
cyclones  are  essentially  convectional  phenomena  on  a  large  scale.  They  occur 
in  seasons  and  regions  where  high  temperatures  prevail ;  they  are  most 
effectively  aided  by  the  abundant  condensation  of  water  vapor  from  air  at  high 
temperatures ;  their  circulation  in  every  way  is  like  that  which  we  should 
expect  would  follow  from  a  convectional  process  on  a  rotating  earth.  Yet  it 
must  be  noted  that  the  essential  fact  on  which  the  belief  in  their  convectional 
character  should  depend  is  not  yet  a  matter  of  direct  observation.  It  has  not 
yet  been  directly  shown  that  the  temperature  of  the  cyclonic  mass  is  higher 
than  that  of  the  surrounding  atmosphere  at  corresponding  altitudes.  If 
observations  on  mountain  peaks  should  in  the  future  show  that  the  cyclonic 
mass  is  not  warmer  than  the  surrounding  air,  the  convectional  theory  of 
tropical  cyclones  would  have  to  be  abandoned  and  some  other  theory  devised 
to  explain  the  phenomena. 

The  student  should  therefore  hold  the  convectional  theory  in  mind  as 
being  well  supported  by  reasonable  evidence,  and  yet  as  still  lacking  the  final 
element  of  direct  demonstration  ;  he  should  remember  the  evidence  that  leads 
to  the  conclusion  here  regarded  as  the  most  probable  one ;  he  should  not 
memorize  the  conclusion  alone.  Recognizing  convection  as  a  process  charac- 
teristic of  gases,  easily  produced  by  experiment  on  small  or  large  scale, 
observable  in  natural  processes  of  various  dimensions,  as  in  the  "  boiling  "  of 
warm  air  over  hot  sandy  surfaces,  in  the  formation  of  cumulus  clouds,  in  the 
movement  of  land  and  sea  breezes,  he  should  appreciate  the  arguments  that 
lead  to  belief  in  the  convectional  origin  of  the  general  circulation  of  the 
atmosphere  between  the  equator  and  poles  and  between  the  continents  and  the 
oceans.  He  might  thus,  indeed,  on  beginning  the  present  chapter,  be  preju- 
diced in  favor  of  a  convectional  origin  for  tropical  cyclones ;  yet  if  he  would 
be  guided  by  the  true  spirit  of  scientific  iiKjiiiry.  lie  must  maintain  an  unsettled 
opinion  as  long  as  the  evidence  is  incomplete  or  contradictory  ;  he  must  adopt 
conclusions  only  where  the  evidence  is  complete  and  convincing;  he  must  over 


CYCLONIC    STORMS    AND    WINDS.  207 

hold  his  mind  open  to  new  evidence,  even  if  it  bring  about  the  abandonment 
of  accepted  beliefs.  He  may,  if  desirable,  quote  the  conclusions  of  others, 
and  if  well  read  he  may  thus  become  widely  informed;  but  he  will  fail  to  gain 
the  best  benefit  that  comes  from  careful  study  if  he  does  not  reach  opinions 
and  conclusions  for  himself,  forming  them  only  as  fast  as  the  evidence  that 
may  support  them  is  clearly  understood. 

The  fact  of  the  occurrence  of  tropical  cyclones  in  certain  regions  and 
seasons  is  not  doubted,  for  the  occurrence  of  their  violent  winds  and  heavy 
rain  is  a  matter  of  repeated  observation.  The  combination  of  the  surface 
winds  to  form  a  vorticular  hurricane  is  demanded  by  numerous  observations 
carefully  studied ;  fifty  years  ago  this  was  a  matter  for  discussion,  but  at 
present  it  need  not  be  questioned.  The  further  interpretation  of  the  surface 
winds  and  the  upper  currents  as  an  inflow  below  and  an  outflow  above,  com- 
bined with  a  whirling  ascent  around  the  center,  is  still  doubted  by  some, 
although  the  evidence  in  its  favor  seems  conclusive  to  most  meteorologists. 
But  when  it  comes  to  the  explanation  of  this  interpretation  as  a  convectional 
overturning,  begun  by  the  heat  of  the  calm  air  in  the  doldrums,  afterwards 
supported  in  good  part  by  the  liberation  of  latent  heat  from  the  condensation 
of  water  vapor,  and  developed  into  true  cyclonic  violence  by  the  deflecting 
force  of  the  earth's  rotation,  —  all  this  is  manifestly  theoretical  to  a  high 
degree  ;  and  belief  in  it  is  warranted  only  after  the  convincing  nature  of  its 
support  is  appreciated. 

There  is  a  correspondence  in  the  progress  of  the  explanations  that  have 
been  given  in  Chapter  VI  for  the  general  circulation  of  the  planetary  winds, 
and  in  the  present  chapter  for  the  occurrence  of  tropical  cyclones,  that  deserves 
brief  mention.  In  the  first  explanation,  the  theory  of  simple  convectional 
interchange  between  the  warm  equator  and  the  cold  poles  was  at  fault,  because 
it  involved  the  occurrence  of  high  pressure  at  the  poles,  while  the  pressure 
there  is  really  low  ;  but  on  supplementing  the  simple  convectional  theory  by 
the  effect  of  the  earth's  rotation,  the  low  pressure  at  the  poles  is  perceived  to 
be  an  essential  feature  of  a  convectional  circulation  on  a  rotating,  globe  ;  and 
the  theory  is  really  strengthened  by  its  survival  of  this  ordeal.  Again,  when 
tropical  cyclones  were  seen  to  possess  a  convectional  inflow,  ascent  and  outflow, 
initiated  in  the  warm  and  moist  air  of  the  doldrums,  no  reason  was  at  first 
perceived  for  their  limitation  to  certain  seasons.  The  doldrums  are  always 
warm  and  moist,  and  are  therefore  always  ready  to  promote  convectional 
storms.  But  here,  again,  the  introduction  of  the  omitted  effect  of  the  earth's 
rotation  fully  accounts  for  the  special  seasonal  and  geographical  distribution 
of  cyclones  in  the  tropical  seas ;  and  the  modified  convectional  theory  is 
therefore  accepted  with  redoubled  confidence.  Hence  in  both  these  explana- 
tions the  simple  convectional  theory  fails  to  account  for  certain  significant 
phenomena  —  the  low  polar  pressures  in  the  first  case  ;  the  peculiar  distribution 


208 


ELEMENTARY    METEOROLOGY. 


of  cyclones  in  the  second  case  —  and  in  both  explanations,  the  introduction  of 
'  e  same  omitted  but  essential  consideration,  namely,  the  fact  that  the  move- 
ments take  place  on  a  rotating  globe,  completely  reconciles  these  diverse 
difficulties.  So  similar  a  progression  of  successful  theoretical  explanation  in 
two  separate  problems  may  be  reasonably  accepted  as  reacting  favorably  on 
both :  indeed,  the  student  may  fairly  measure  his  appreciation  of  the  arguments 
that  have  been  employed  in  these  chapters  by  the  increase  of  his  confidence  in 
their  conclusions  after  their  similarity  is  recognized. 

EXTRA-TROPICAL  CYCLONES. 

232.  Comparison  of  tropical  and  extra-tropical  cyclones.  On  turning  to 
the  cyclones  of  the  temperate  latitudes,  we  find  many  features  in  which  they 
resemble  those  of  the  torrid  zone,  and  certain  other  features  in  which  they 


FIG.  (il. 

differ.  Their  fundamental  resemblance  to  tropical  cyclones  is  seen  in  their 
incurving  vorticular  winds,  whirling  to  the  left  in  this  hemisphere,  to  the  ri.^lit 
in  the  other,  around  a  moving  center  of  low  pressure.  Numerous  charact«-i -istu; 


CYCLONIC    STORMS    AND    WINDS.  209 

examples  of  such  storms  are  found  in  the  magnificent  Atlas  of  daily  weather 
maps  of  the  North  Atlantic  ocean  for  the  year  beginning  August,  18^, 
published  by  the  British  Meteorological  Council.  Fig.  61  represents  the 
isobars  for  every  tenth  of  an  inch  around  one  of  these  storms  for  noon  of 
January  14,  1883.  The  storm  center  had  a  pressure  of  less  than  28.00,  or  an 
inch  and  a  half  below  the  normal  for  the  place  and  season,  as  given  on  Chart 
V;  it  was' formed  by  the  union  of  several  subordinate  cyclonic  centers,  which 
all  coalesced  near  the  center  of  the  North  Atlantic  low  pressure  area  of  winter, 
producing  a  storm  of  unusual  severity.  The  contrast  between  the  gentle  and 
uniform  gradients  of  the  torrid  zone  and  the  strong  and  variable  gradients  of 
the  temperate  zone,  as  here  illustrated,  is  very  striking.  Many  similar 
examples  may  be  found  in  the  continuation  of  this  Atlas  by  the  marine 
observatories  of  Germany  and  Denmark,  as  well  as  in  the  weather  maps  of 
various  countries  (Sect.  325).  As  the  surface  winds  obliquely  approach  the 
storm  center  from  all  sides,  an  upward  escape  must  be  inferred  for  them ; 
and  as  in  the  case  of  the  tropical  cyclones,  this  is  confirmed  by  the  occurrence 
of  extended  clouds  and  heavy  rainfall,  and  by  the  forward  outflow  of  cirrus 
streamers  aloft. 

As  with  tropical  cyclones,  the  cyclones  of  our  latitudes  vary  in  intensity 
with  the  depression  of  the  barometer  at  the  center ;  and  here  as  there  the 
greater  part  of  the  depression  is  to  be  regarded  as  the  effect  of  the  centrifugal 
forces  of  the  revolving  winds  ;  but  the  greater  part  of  these  forces  in  a  tropical 
cyclone  arises  from  the  true  centrifugal  force  of  the  wind's  rotation  around 
the  storm  center,  and  is  only  in  a  lesser  proportion  due  to  the  deflecting  force 
of  the  earth's  rotation  ;  while  this  relation  is  reversed  in  extra-tropical  cyclones, 
where  the  deflecting  force  is  greater  than  the  true  centrifugal  force  of  the 
whirl,  because  of  the  higher  latitude  -in  which  these  storms  occur.  The  central 
region  of  exceptionally  low  pressure  and  very  steep  gradients  in  tropical 
cyclones  is  relatively  small,  because  a  strong  centrifugal  force  is  produced 
only  when  the  winds  are  whirling  on  a  short  radius  ;  the  low-pressure  area  of 
our  cyclones  is  much  larger  and  the  gradients  have  a  tolerably  strong  value  for 
some  distance  around  the  center,  because  the  depression  of  the  isobars  depends 
rather  on  the  latitude  of  occurrence  than  on  the  distance  of  the  wind  from  the 
storm  center  ;  for  this  reason  there  is  less  concentration  of  violence  close  to  the 
center,  and  the  calm  and  clear  central  space  or  eye  is  seldom  sharply  developed, 
although  it  is  not  uncommon  to  discover  a  gradual  weakening  or  failing  of  the 
winds,  and  sometimes  even  an  imperfect  breaking  away  of  the  clouds,  as  the 
central  area  passes  over  the  observer.  The  form  of  tropical  cyclones,  as  defined 
by  their  isobaric  lines,  is  nearly  circular.  Our  cyclones  are  as  a  rule  less 
symmetrical,  and  their  isobars  are  often  elongated  into  an  oval  form.  In  the 
eastern  United  States,  the  longer  axis  of  the  oval  trends  northeast,  making  a 
trough-like  depression  between  the  high-pressure  area  over  the  tropical  North 


210  ELEMENTAL  Y     MK  TKOKOLOGY. 

Atlantic  and  the  winter  high-pressure  area  of  North  America.  In  the  Nor.li 
Atlantic,  the  lowest  pressure  of  the  cyclone  is  commonly  found  south  of  tin- 
center  of  the  outer  isobaric  ovals,  thus  giving  steep  gradients  south  of  the 
center  and  weak  gradients  north  of  it;  this  is  due  to  the  occurrence  of 
prevailing  high  pressures  about  the  Azores  and  low  pressures  about  Iceland. 
In  the  torrid  zone,  where  the  isobaric  chart  for  January  or  July  shows  a 
relatively  uniform  distribution  of  pressure,  these  causes  of  irregularity  are 
absent. 

Extra-tropical  cyclones  are  of  frequent  occurrence,  greatly  disturbing  the 
flow  of  the  westerly  winds.  If  either  hemisphere  could  be  seen  by  an 
observer  far  above  the  earth,  he  would  discover  an  almost  continuous  proces- 
sion of  white  cyclonic  cloud  sheets  spiralling  around  the  pole  in  the  middle 
or  higher  latitudes,  crossing  continents  and  oceans,  and  with  greater  frequency, 
greater  intensity  and  greater  progressive  velocity  in  winter  than  in  summer. 
Although  often  attaining  destructive  violence  at  sea,  the  winds  of  extra- 
tropical  cyclones  commonly  rise  only  to  moderate  or  strong  gales  on  land, 
where  they  are  to  be  less  dreaded  than  welcomed,  on  account  of  the  supply  of 
rainfall  that  they  bring,  so  important  in  rendering  large  continental  areas 
habitable.  Tropical  cyclones,  on  the  other  hand,  are  nearly  always  of 
devastating  violence  ;  they  are  so  greatly  to  be  dreaded  that  it  is  fortunate  they 
are  relatively  rare.  Far  from  having  an  increased  strength  in  winter,  they 
are  produced  only  in  the  everlasting  summer  of  the  doldrums  or  of  the 
adjacent  weakened  trades.  The  extra-tropical  cyclones  are  associated  with 
travelling  areas  of  high  pressure,  called  anticyclones,  of  which  a  fuller 
account  will  be  given  later.  Tropical  cyclones  appear  to  be  surrounded  by  a 
faint  pericyclonic  ring  of  slightly  increased  pressure,  already  described ;  but 
except  for  this,  the  distribution  of  pressure  over  the  torrid  zone  is  remarkably 
equable.  '  The  great  difference  in  the  values  of  the  monthly  range  of  baro- 
metric pressure  in  the  torrid  and  temperate  zones  (Sect.  105)  is  thus  explained. 

The  two  classes  of  storms  differ  in  their  direction  and  regularity  of 
progress.  The  extra-tropical  cyclones  generally  move  in  an  easterly  course, 
turned  slightly  toward  the  pole,  as  if  circling  around  it ;  but  they  sometimes 
turn  irregularly  to  one  side  or  the  other,  and  occasionally  even  move  back- 
wards. Two  storms  sometimes  approach  and  merge  into  one  :  or  a  single 
storm  develops  a  subordinate  or  secondary  center  of  low  pressure,  which  may 
increase  in  importance  and  duration  over  its  parental  storm.  A  number  of 
characteristic  tracks  of  storm  centers  for  our  hemisphere  are  shown  in  Fig.  62 
by  Loomis.  The  tracks  of  cyclones  of  tropical  origin  here  included  may  !•<• 
easily  distinguished  from  the  others  by  their  recurved  course  near  the  tropic. 
The  velocity  of  progression  is  also  different  in  the  two  classes;  in  our 
latitudes  it  amounts  to  from  fifteen  to  thirty  miles  an  hour;  this  being  from 
two  to  fourfold  the  progressive  velocity  of  tropical  cyclones  while  yet  in  the 


CYCLONIC    STORMS    AND    WINDS. 


torrid  zone  (Sect.  240) ;  but  on  entering  the  temperate  zone  and  recurving 
towards  the  east,  the  tropical  storms  take  on  all  the  features  of  cyclones  that 
have  originated  there.  Storms  of  the  two  classes  sometimes  merge  into  a 
single  center,  as  if  their  motions  before  union  were  entirely  accordant. 

233.   Unsymmetrical    form    of    extra-tropical    cyclones.      One    of    the 

strongest  contrasts  between  the  two  classes  of  storms  is  found  in  the  distribu- 
tion of  temperature,  clouds  and  rainfall,  with  respect  to  the  center  of  low 


212 


ELEMENTARY    METEOROLOGY. 


pressure.  The  cause  of  this  contrast  may  be  readily  understood  by  comparing 
the  surroundings  of  cyclones  in  the  two  zones.  The  oceanic  area  of  the  torrid 
zone  is  a  vast  region  of  remarkable  uniformity;  for  hundreds  of  miles  on  all 
sides  of  a  cyclonic  center  the  temperature  and  humidity  of  the  air  vary 
but  little.  Inflowing  currents  from  all  sides  are  nearly  alike  as  to  heat 
and  moisture;  isotherms  in  tropical  cyclones  may  coincide  closely  with 
isobars,  and  both  approach  a  circular  form.  The  areas  of  lower  clouds,  of 


FIG.  64. 

upper  clouds,  and  of  rainfall  extend  almost  symmetrically  on  all  sides  of  the 
storm  center.  Thus  tropical  cyclones  are  remarkably  simple  and  regular  in 
their  form  and  in  the  distribution  of  their  parts  about  the  center. 

Consider  now  the  case  of  an  extra-tropical  cyclone  moving  across  the  Ohio 
valley  in  winter  time;  such  a  one,  for  example,  as  that  of  February  19,  1.ss  I 
(Fig.  64).  To  the  south  and  east  of  the  cyclonic  center  the  atmosphere  is 
relatively  mild  and  damp  over  the  Gulf  of  Mexico  and  the  warm  waters  of 
the  Gulf  Stream  in  the  western  Atlantic.  To  the  north  and  west  the  cold, 
snow-covered  plains  of  the  continental  interior  extend  for  over  a  thousand 


CYCLONIC    STORMS    AND    WINDS. 


213 


miles,  unbroken  by  mountain  ranges,  and  surmounted  by  a  clear,  cold  and  dry 
atmosphere.  The  inflowing  southerly  and  easterly  winds  that  enter  the  front 
of  the  storm  area  become  cooled  as  they  advance  into  higher  latitudes  and 
over  the  cold  surface  of  the  land,  and  still  more  as  they  begin  their  oblique 
ascent  around  the  storm  center;  the  cloud  and  rain  areas  are  thus  greatly 
extended  to  the  south  and  east  of  the  center  of  low  pressure.  The  cool,  dry 
winds  from  the  west  and  northwest  become  for  a  time  warmer  and  dryer  as 
they  advance  obliquely  towards  the  storm  center,  because  they  move  over 
lands  of  higher  temperature  than  that  o'f  their  source,  and  because  they  enter 
latitudes  where  insolation  is  more  effective  in  warming  them;  and  these 
causes  of  increased  heat  and  dryness  must  be  overcome  by  the  cooling  of 
ascent,  as  the  winds  whirl  around  the  center  of  low  pressure,  before  any  clouds 
and  rain  can  be  formed  in  them.  The  cloud  and  rain  areas  are  therefore 
much  less  extended  to  the  west  than  to  the  east  of  our  cyclonic  storms.  The 
dissimilar  temperatures  at  the  source  of  these  winds  naturally  result  in  a 
strong  distortion  of  the  isotherms  within  the  cyclonic  area;  they  are  carried 


5 


. 


FIG.  65. 


FIG.  66. 


northward  in  front,  and  southward  in  the  rear  of  the  storm,  as  appears  with 
exceptional  distinctness  in  the  illustration  given  above.  Furthermore,  the 
overflowing  cirrus  clouds,  radiating  in  all  directions  somewhat  eccentrically 
over  the  storm,  extend  their  plumes  much  further  in  advance  of  the  center 
than  backwards  from  it,  as  might  be  expected  from  the  occurrence  of  these 
disturbances  in  a  latitude  where  the  upper  currents  of  the  atmosphere  move 
eastward  at  a  high  velocity.  The  feathery  cirrus  streamers  are  often  visible 
a  day  or  more  before  the  arrival  of  the  lower  clouds.  At  the  higher  cirrus 
level,  five  or  more  miles  above  the  sea,  the  whirling  motion  so  apparent  in  the 
lower  winds  is  reduced  to  a  general  eastward  drift,  varying  but  little  from  the 
velocity  and  direction  of  the  higher  currents  of  the  general  winds. 

The  results  of  systematic  observations  on  the  winds  and  cirrus  clouds  at 
Blue  Hill,  Mass.,  within  the  area  of  cyclonic  storms,  are  presented  in  Figs.  65 
and  66  r1  the  rectangles  in  these  diagrams  being  five-degree  "squares"  of 

1  For  a  fuller  account  of  Figs.  65,  66,  68,  69,  see  an  article  by  H.  H.  Clayton,  in  the 
American  Meteorological  Journal,  August,  1893. 


214  ELEMENTARY    MKTKOKOLOGY. 

latitude  and  longitude.  Fig.  65  shows  the  inflowing  vorticular  winds  on  the 
summit  of  the  hill  with  respect  to  the  cyclonic  center  ;  the  irregular  form  of 
our  cyclones  almost  disappearing  in  this  combination  of  many  examplea. 
Fig.  66  shows  the  movement  of  the  cirrus  clouds  above  the  cyclonic  area ;  the 
deflections  from  the  prevailing  eastward  movement  with  respect  to  the 
cyclonic  center  being  such  as  to  indicate  a  somewhat  irregular  outflow  toward 
the  margin  of  the  region  ;  the  stronger  outward  movement  to  the  north  being 
attributed  to  the  relatively  high  temperatures  on  the  east  of  the  storm. 

In  the  frequent  mention  of  whirling  and  ascent  in  our  cyclonic  winds  that 
will  be  met  with  in  succeeding  paragraphs,  these  statements  regarding  the 
deformation  of  the  cyclonic  whirl  must  be  borne  in  mind.  The  comparative 
symmetry  of  the  winds  and  clouds  around  the  vortex  of  tropical  cyclones  is 
not  observed  in  our  latitudes.  The  whirling  is  distinct  enough  in  the  lower 
winds,  but  the  rotary  motion  seems  to  be  brushed  forward  and  obliterated  at 
the  height  of  the  cirrus  clouds.  The  ascensional  movement  about  the  central 
region  is  of  course  in  no  cases  vertical,  but  always  compounded  with  the 
whirling  or  advancing  movement  of  the  winds. 

The  want  of  symmetry  is  so  well  marked  in  the  stronger  winter  cyclonic, 
storms  of  our  latitudes  that  some  meteorologists  are  disposed  to  exclude  them 
from  the  cyclonic  class.  This,  however,  seems  to  be  hardly  warranted  ;  for 
the  peculiarities  are  all  such  as  would  result  from  the  occurrence  of  whirling 
storms  in  regions  of  varied,  instead  of  uniform  surroundings,  and  in  a  rapidly 
moving,  instead  of  in  a  relatively  quiet  portion  of  the  earth's  atmosphere. 
There  does  not  seem  to  be  a  fundamental  difference  in  the  movement  of  the 
winds  in  the  two  classes  of  cyclones,  however  different  their  causes  may  be 
found.  The  storms  of  the  two  zones  not  only  exhibit  a  distinct  relationship  ; 
those  of  the  torrid  zone  gradually  lose  their  symmetry  when  they  advance 
into  the  temperate  zone,  and  take  on  all  the  unsymmetrical  features  of 
the  storms  of  extra-tropical  latitudes.  Numerous  instances  of  this  trans- 
formation may  be  found  in  the  Atlases  of  the  North  Atlantic,  mentioned 
in  Section  232. 

234.  The  center  of  extra-tropical  cyclones.  The  clear  central  eye  of 
tropical  cyclones  is  not  often  displayed  in  the  cyclones  of  our  latitudes, 
e«i>ecially  on  land.  Our  cyclonic  winds  decrease  somewhat  on  the  weaker 
gradients  near  the  center,  but  the  clouds  do  not  often  break  away,  unless  the 
surrounding  winds  are  of  exceptional  violence.  It  has  been  concluded  from 
this  that  the  spirally  inflowing  lower  winds  do  not  gain  so  great  a  centrifugal 
force  as  to  prevent  their  being  drawn  into  the  central  district  of  low  pressuiv, 
which  is  formed  by  the  more /violent  whirling  of  the  winds  at  higher  levels. 
In  this  respect  the  surface/winds  of  our  cyclones  may  be  compared  to  tlin 
surface  member  of  the  planetary  circulation,  which  flows  obliquely  towards 


, 

.1  (  -  " 


CYCLONIC    STORMS    AND    WINDS.  216 

the  low-pressure  area  of  the  polar  regions  ;  the  low  pressure  there  having 
been  caused  by  the  much  more  active  whirling  of  the  upper  and  middle 
members  of  the  circulation  (Sect.  136).  Such  a  relation  might  be  well 
expected  in  our  cyclones  on  land,  because  the  lower  winds  are  there  held  down 
to  moderate  velocities  by  the  greater  resistances  that  oppose  them  ;  and  they 
are  therefore  more  subject  to  the  control  of  the  central  low  pressure  as  deter- 
mined by  the  stronger  winds  at  greater  altitudes. 

235.  Control  of  weather  by  cyclones.     It  is  manifest  from  the  association 
of  areas  of  cloud  and  rainfall  with  cyclonic  centers,  from  the  deformation  of 
the  isothermal  lines  before  and  behind  them,  and  from  their  frequent  occur- 
rence and  their  generally  regular  and  rapid  advance  in  an  eastward  course, 
that  the  changes  of  weather  in  the  temperate  zone  must  be  largely  controlled 
by  the  passage  of  cyclonic  storms.     As  they  draw  near,  the  sky  becomes  over- 
clouded ;  the  prevailing  westerly  wind  falls  away,  and  is  succeeded  by  a  wind 
from  some    easterly  direction,   faint  at  first,   but   increasing  as  the  falling 
barometer  and  heavier  clouds  and  rain  betoken  the  approach  of  the  storm 
center;  the  temperature  rising,  if  the  center  passes  north  of  the  observer, 
until  the  wind  veers  through  the  south  to  the  west,  bringing  a  cool  or  cold 
current  with  rising  barometer  and  clearing  sky :   the  temperature  remaining 
relatively  low,  and  the  wind  backing  from  the  east  through  the  north  to  the 
west,  if  the  center  passes  south  of  the  observer.     These  changes  are  of  great 
practical  importance  ;  they  will  be  more  fully  considered  in  Sections  243  and 
315 :  but  we  have  first  to  examine  the  origin  and  movement  of  our  cyclonic 
storms. 

236.  Non-con vectional  origin  of  extra-tropical  cyclones.     When  we  come 
to  seek  the  cause  of  the  cyclones  of  our  latitudes,  it  is  seen  that  one  of  their 
characteristics  distinguishes  them  strongly  from  the  cyclones  of  the  torrid 
zone.       This    is   their   greater   frequency   and   intensity   in   winter   than   in 
summer. 

With  this  contrast  in  mind,  we  must  inquire  whether  there  is  good  reason 
to  think  that  extra-tropical  cyclones  may,  like  the  tropical  cyclones,  be 
regarded  as  convectional  storms,  to  which  a  whirling  motion  is  given  by  the 
deflecting  force  of  the  earth's  rotation.  This  question  must  at  present  be 
answered  most  probably  in  the  negative,  in  spite  of  the  many  other  likenesses 
between  the  two  classes  of  storms.  When  our  cyclones  are  traced  back  to 
their  beginning  at  one  place  or  another,  on  land  or  water,  there  is  not  found, 
even  in  summer,  any  such  persistent  and  distinct  indication  of  instability  and 
convection  as  appears  in  the  doldrums,  where  the  tropical  cyclones  begin. 
More  than  this,  the  occurrence  of  extra-tropical  cyclones  with  increased 
intensity  in  the  winter  season  forbids  the  supposition  that  they  arise  as  a  rule 


216  ELEMENTARY    METEOROLOGY. 

from  the  convectional  overturning  of  an  unstable  mass  of  air.  If  reference  is 
made  to  the  sections  describing  atmospheric  instability  and  convection  (52-«>4), 
it  will  be  seen  that  the  winter  season  is  precisely  the  time  when  convection  is 
most  unlikely.  In  that  season  there  is  a  relatively  slow  vertical  decrease  of 
temperature,  while  directly  the  opposite  condition  is  necessary  for  instability. 
Moreover,  in  winter,  when  the  lower  air  is  prevailingly  cold,  the  amount  of 
latent  heat  liberated  by  the  condensation  of  vapor  in  ascending  currents  of  air 
is  small  compared  to  that  liberated  by  an  equal  ascent  of  warmer,  moister  air 
in  the  summer  time  (Sect.  197).  Latent  heatr  which  has  been  shown  to  plaj 
so  essential  a  part  in  the  working  of  tropical  cyclones,  is  not  so  effective  an 
aid  to  storm  action  in  winter  as  in  summer ;  and  yet  it  is  in  winter  time  that 
our  cyclones  possess  their  greatest  violence.  Spontaneous  convectional  action, 
therefore,  does  not  seem  to  be  the  chief  cause  of  extra-tropical  cyclones.  It 
may  be  that  some  of  our  cyclones,  especially  in  summer  and  on  land,  are  of 
convectional  beginning ;  it  surely  is  not  wise  in  the  present  state  of  meteor- 
ology to  exclude  convectional  action  from  all  share  in  beginning  and  maintain- 
ing these  storms  ;  it  is  certain  that  the  latent  heat  liberated  from  their  rains 
aids  their  action  ;  yet  it  would  seem  prudent  to  search  for  some  other  cause 
of  their  origin  and  action  than  convection  ;  to  look  for  some  cause  whose  value 
shall  be  greatest  in  that  season  when  the  cyclones  have  their  greatest 
frequency  and  activity. 

237.  Origin  of  extra-tropical  cyclones  as  eddies  in  the  circumpolar  winds 
of  the  terrestrial  circulation.  The  only  cause  of  this  kind  that  has  yet  been 
discovered  is  the  general  circulation  of  the  terrestrial  winds  around  tae  poles. 
It  has  already  been  explained  that  the  upper,  middle  and  lower  members  of 
this  circulation  move  in  a  general  eastward  direction  in  the  middle  and  higher 
latitudes,  and  with  a  considerable  velocity ;  it  has  also  been  shown  that,  on 
account  of  the  increased  temperature  gradient  between  the  equator  and  poles 
in  the  winter  hemisphere,  it  will  be  in  that  hemisphere  that  the  general 
circumpolar  winds  possess  the  greatest  velocity  ;  while  in  the  summer  hemi- 
sphere, where  the  contrast  of  equatorial  and  polar  temperatures  is  reduced,  the 
circumpolar  winds  blow  less  rapidly.  It  has  therefore  been  suggested  that 
our  extra-tropical  cyclones  are  not  spontaneous  convectional  disturbances,  but 
secondary  eddies  driven  by  the  general  winds. 

Reference  should  now  be  made  to  Section  131,  in  which  the  eastward 
course  of  the  poleward  overflow  from  the  expanded  air  above  the  equator  was 
explained.  It  was  there  stated  that  the  eastward  course  of  the  wind  was 
determined  by  a  balance  between  the  forward-acting  acceleration  (the  component 
of  the  poleward  gravitative  force  on  tin-  strong  upper  ^nidii-nts  not  overcome 
by  the  deflecting  force  of  the  earth's  rotation)  and  the  backward-acting  resist- 


CYCLONIC    STORMS    AND    \VIM»S.  217 

ances  ;  but  the  character  of  the  resistances  was  not  then  particularly  inquired 
into.  They  should  now  be  considered  more  carefully. 

The  resistances  excited  by  the  variation  in  the  velocity  or  direction  of  the 
successive  strata  of  the  atmosphere  must  be  very  small,  yet  it  has  been  sug- 
gested that  these  may  be  sufficient  to  produce  undulations  in  adjacent  currents 
analogous  to  those  by  which  certain  kinds  of  clouds  have  been  explained 
(Sect.  203),  but  of  much  greater  size.  The  short-circuit  return  of  the  equatorial 
overflow  as  it  advances  obliquely  toward  the  pole  over  the  converging  meridians, 
explained  in  Section  141,  may  cause  local  crowding  or  congestion.  The 
inequality  of  the  poleward  gradients  in  place  and  time  must  result  in  some 
irregularity  in  the  winds  around  the  poles.  Under  favorable  conditions,  these 
various  causes  may  give  rise  to  entangling  motions  of  the  upper  winds,  from 
which  great  eddies  in  the  lower  winds  could  be  derived.  The  resistances  of 
continents  and  mountains  must  contribute  in  some  degree  to  the  disorder  of 
the  general  winds ;  yet  only  in  a  subordinate  way,  if  we  may  judge  by  the 
occurrence  of  about  as  many  cyclonic  storms  in  the  southern  as  in  the  northern 
temperate  zone.  In  whatever  way  the  disturbances  are  caused  by  the  general 
winds,  it  is  natural  that  they  should  be  more  frequent  in  the  faster-moving 
circumpolar  whirl  of  the  middle  and  higher  latitudes  than  in  the  slower-moving 
winds  of  the  torrid  zone ;  and  that  the  middle  and  higher  latitudes  should 
witness  more  stormy  disturbances  in  their  winter  season  when  their  general 
winds  are  running  rapidly,  than  in  the  summer  time,  when  their  general  circula- 
tion is  somewhat  relaxed.  All  these  considerations  have  in  recent  years 
turned  the  tide  of  opinion  against  accepting  a  purely  convectional  origin  for 
extra-tropical  cyclones,  and  directed  it  towards  ascribing  their  origin  in  greater 
part  to  eddies  in  the  general  circumpolar  winds. 

A  manifest  difficulty  in  the  way  of  this  explanation  of  our  cyclones  is  their 
long  endurance.  Many  cyclones  have  been  traced  a  third,  a  half,  or  even  a 
larger  share  of  the  way  around  the  north  temperate  zone  ;  and  it  is  difficult  to 
understand  how  they  could  survive  so  long  if  produced  as  suggested  above. 
At  present,  no  satisfactory  explanation  of  this  difficulty  can  be  given. 

238.  Anticyclones.  Areas  of  high  pressure,  called  anticyclones,  are  of  as 
common  occurrence  as  cyclonic  areas  of  low  pressure  in  extra-tropical  latitudes. 
The  two  are  intimately  associated  and  usually  increase  or  decrease  in  intensity 
together ;  the  anticyclones  alternating  somewhat  regularly  with  the  cyclones 
in  the  procession  of  disturbances  that  marches  around  the  poles.  Just  as  the 
cyclones  frequently  merge  into  the  larger  areas  of  low  pressure  that  lie  over 
the  oceans  in  high  latitudes  during  the  winter,  so  the  anticyclones  often  com- 
bine with  the  areas  of  high  pressure  that  lie  over  the  northern  continents  in 
the  colder  months.  Whatever  explanation  is  given  of  one  of  these  classes  of 
phenomena  should  throw  light  on  the  origin  of  the  other  as  well. 


218 


ELEMENTARY    METEOROLOGY. 


Fig.  67  illustrates  several  of  the  characteristic  features  of  anticyclones. 
It  represents  the  condition  of  the  atmosphere  at  7  A.M.  over  the  eastern 
United  States  on  January  25,  1880.  The  central  pressures  are  over  30.1  ; 
the  central  gradients  are  weak  and  are  directed  outward  on  all  sides  ;  the  air 
is  calm  or  gently  moving  in  the  central  region,  and  flows  slowly  outward  in  a 
right-handed  spiral  over  the  marginal  area.  The  sky  is  generally  clear  ;  the 
hour  of  observation  being  too  early  for  the  formation  of  diurnal  clouds,  such 
as  often  arise  in  an  anticyclonic  area  during  the  middle  of  the  day,  especially 


Fie.  07. 

in  summer.  The  temperature  of  the  central  clear  area  is  lower  than  that  of 
the  surrounding  districts,  even  lower  than  to  the  north ;  because  of  the  free 
radiation  from  the  earth  through  the  clear  anticyclonic  air,  while  the  greater 
cloudiness  of  the  surrounding  districts  has  diminished  their  nocturnal  fall  <  i 
temperature. 

In  winter,  when  insolation  is  brief  and  weak,  the  surface  temperatures  in 
anticyclonic  areas  are  prevailingly  low,  especially  on  land,  by  reason  of  the 
raj. id  cooling  of  the  ground  by  radiation.  In  summer  time,  they  are  low  at 
night  for  the  same  reason;  but  they  are  then  relatively  high  in  the  day-time, 
on  account  of  the  strength  and  long  duration  of  the  insolation  that  enters 


CYCLoNK      SToKMS    AND    WINDS. 


219 


through  their  clear  sky.  In  both  seasons,  the  lower  air  of  anticyclones  has  a 
relatively  large  diurnal  range  of  temperature  and  a  distinctly  marked  diurnal 
period  in  the  velocity  of  the  wind. 

The  outflow  of  the  lower  wind  from  anticyclonic  areas  is  shown  in  Fig.  68, 
which  represents  the  average  direction  and  velocity  of  the  winds  at  Blue  Hill, 
Mass.,  around  an  anticyclonic  center.  This  outward  movement,  coupled  with 
the  prevailingly  clear  and  dry  condition  of  the  atmosphere  leads  to  the 
belief  that  areas  of  high  pressure  are  regions  of  inflowing  air  aloft  and  slow 
down-settling  about  the  central  area.  In  this,  as  in  so  many  other  features, 
they  are  the  opposite  of  areas  of  low  pressure,  or  cyclones ;  hence  the  name, 
anticyclone,  suggested  by  Galton  in  1863.  The  occurrence  of  an  upper  inflow 
is  confirmed  by  the  observations  of  such  cirrus  clouds  as  wander  near  them. 
The  average  direction  and  velocity  of  cirrus  cloud  movement  over  Blue  Hill 
within  anticyclonic  areas,  Fig.  69,  are  changed  from  the  general  eastward 


6 

~  -~~ 

/  .. 

>* 
s^ 

N 

• 

:'S 

-•* 

—  » 

\ 
N 

N 

\ 

\\ 

•  V 

\ 

\ 

• 

:\ 

i 

\ 

r^... 

•£••• 

/ 

\\ 

V' 

2. 


N 


N 


7 


FIG.  68. 


FIG.  69. 


movement  of  the  upper  clouds  in  such  a  way  as  to  indicate  a  somewhat 
irregular  inward  movement  with  respect  to  the  advancing  anticyclonic  center. 
Further  confirmation  of  the  descent  of  air  within  anticyclones  will  be  found  in 
Section  249. 

If  anticyclones  are  convectional  phenomena,  in  which  the  air  descends 
spontaneously  by  its  own  weight,  they  must  as  a  whole  have  a  relatively  low 
temperature ;  the  converse  of  convectional  cyclones,  in  which  the  temperature 
of  the  mass  must  be  relatively  high.  According  to  this  theory,  the  warmth  of 
cyclones  would  be  gained  chiefly  in  the  lower  air,  where  insolation  is  best 
applied  to  raising  the  temperature  of  the  atmosphere  ;  and  the  low  temperature 
of  anticyclones  would  be  referred  to  radiation  from  the  upper  air  :  but  as  this  is 
a  relatively  inactive  process,  the  cyclones  would,  as  a  rule,  take  the  initiative, 
and  the  anticyclones  would  follow  as  consequences  of  cyclonic  overflow  aloft. 
The  anticyclones  would  then  be  explained  as  the  overlapping  of  two  or  more 
adjacent  pericyclonic  rings. 

If,  on  the  other  hand,  cyclonic  and  anticyclonic  disturbances  are  produced 
by  the  irregular  flow  of  the  general  winds,  it  is  probable  that  these  disturbances 


220  ELEMENTARY    METEOROLOGY. 

would  originate  in  the  higher  regions  of  the  atmosphere,  where  the  winds 
blow  much  faster  than  near  the  earth's  surface.  The  differences  of  pressure 
produced  at  high  altitudes  would  be  felt  down  to  sea-level ;  and  as  the  lower 
winds  move  with  comparative  slowness,  they  would  be  governed  by  the 
gradients  thus  imposed  on  them  by  the  irregular  movements  of  the  upper 
winds.  According  to  this  theory,  an  area  of  high  pressure  or  anticyclone 
would  be  perceived  at  sea-level  beneath  a  district  where  the  upper  currents 
crowd  together ;  and  an  area  of  low  pressure  or  a  cyclonic  storm  would  be 
developed  beneath  a  region  where  the  upper  currents  are  somewhat  divergent. 
Between  the  areas  of  high  and  low  pressure  thus  produced  there  would  be  a 
constant  play  of  the  lower  winds  ;  they  would  run  out  from  the  centers  of 
high  pressure  and  their  outflow  would  be  supplied  by  a  slow  descent  from 
aloft,  and  such  areas  would  be  consequently  dry  and  free  from  lower  clouds ; 
the  lower  winds  would  run  towards  and  whirl  around  the  centers  of  low 
pressure,  gathering  vapor  on  the  way,  obliquely  ascending  there  and  becoming 
cloudy  and  rainy.  As  the  crowding  of  the  upper  currents  appears  to  be  the 
more  effective  of  the  two  processes  here  concerned,  the  anticyclones  would, 
according  to  this  explanation,  take  the  initiative,  and  the  cyclones  would  be 
regarded  as  relatively  secondary. 

239.  Test  of  the  theories  of  extra-tropical  cyclones  and  anticyclones. 
Some  impartial  test  of  the  two  theories  by  which  stormy  disturbances  in  the 
general  winds  are  explained  is  now  needed ;  and  an  admirable  one  has  been 
presented  by  Dr.  Hann  of  Vienna  in  his  studies  of  the  temperatures  prevailing 
in  cyclones  and  anticyclones  as  recorded  at  the  mountain  observatories  of  the 
Alps.  In  order  to  appreciate  the  force  of  his  arguments,  we  must  first  review 
the  contrasted  consequences  of  the  two  theories  under  discussion. 

If  cyclones  and  anticyclones  are  convectional  phenomena,  the  former  must 
be  regions  of  relatively  high  temperature,  and  the  latter  of  relatively  low 
temperature,  when  compared  with  one  another  or  with  the  surrounding  atmo- 
sphere. The  isobaric  surfaces  of  the  warm  cyclones  must  be  held  apart,  in 
order  to  produce  the  inward  gradients  below  and  the  outward  gradients  abmv, 
as  explained  in  Section  93,  although  the  shape  of  the  upper  isobaric  surfaces 
may  be  depressed  by  the  centrifugal  forces  of  the  whirling  winds,  as  explained 
for  the  case  of  tropical  cyclones  in  Section  229.  The  isobaric  surfaces  of  the 
cold  anticyclones  must  be  pressed  closer  together  in  order  to  produce  the 
centripetal  gradients  aloft,  where  the  winds  flow  inward,  and  the  centrifugal 
gradients  below,  where  the  winds  move  outward.  As  both  the  cyclones  and 
anticyclones  endure  for  days  or  weeks  together,  it  follows  that  the  atmosphere 
from  which  their  indrafts  are  supplied  should  be  relatively  warm  and  moist 
in  its  lower  levels,  in  order  to  produce  the  instability  <>n  which  these  conwr- 
tional  disturbances  are  supposed  to  depend.  In  the  case  of  the  cyclones  the 


CYCLONIC    STORMS    AND    WINDS. 


'2'21 


surface  air  must  be  warm  enough,  or  warm  and  moist  enough,  to  maintain  a 
higher  temperature  than  that  of  the  surrounding  air  through  Avhich  it  rises, 
in  spite  of  its  cooling  by  expansion  in  ascent.  In  the  case  of  the  anticyclones 
the  upper  air  must  be  cold  enough  to  remain  at  a  lower  temperature  than  that 
of  the  surrounding  air  through  which  it  settles  down,  in  spite  of  the  increase 
of  temperature  by  compression  during  descent. 

If  cyclones  and  anticyclones  are  driven  eddies,  forced  to  move  by  the 
energy  of  the  general  circumpolar  winds,  no  such  instability  need  be  assumed. 
The  air  of  a  driven  eddy  near  a  street  corner  in  a  blustering  wind  is  not 
necessarily  warmer  and  lighter  than  the  air  through  which  its  whirling 
currents  are  raised ;  it  may  be  heavier  than  its  surroundings,  as  is  the  dust 
that  it  bears  aloft,  and  owe  its  ascensional  motion  to  some  external  force 
stronger  than  its  own  weight,  instead  of  rising  spontaneously  like  a  hot  desert 
whirlwind.  In  like  manner,  the  air  of  an  extra-tropical  cyclone  in  the  boister- 
ous circumpolar  circulation  is  not  necessarily  lighter  than  its  surroundings ; 
the  air  as  well  as  the  clouds  that  it  sustains  may  be  heavier  than  its  surround- 
ings ;  it  may  be  driven  up  by  some  greater  external  force,  instead  of  rising 
spontaneously  like  the  air  of  a  warm  and  moist  tropical  cyclone.  So 
conversely  with  anticyclones ;  if  they  do  not  sink  by  their  own  weight,  but 
are  crowded  down  by  the  accumulation  of  higher  currents  above  them,  the 
heat  that  they  acquire  by  compression  may  make  them  even  lighter,  volume 
for  volume,  than  the  surrounding  air. 

The  contrasted  consequences  of  these  rival  theories  may  be  more  clearly 
illustrated  by  the  following  figures.  Fig.  70  presents  the  sequence  of  changes 
of  temperature  involved  in  a  spontaneous  convectional  circulation.  The 
relatively  warm  and  moist  lower  air,  with  tempera- 
ture A,  cools  rapidly  at  the  ordinary  adiabatic  rate, 
AB,  until  its  dew  point  is  reached  and  condensation 
of  vapor  begins.  Cooling  is  then  retarded  to  the 
slower  rate,  BC ;  but  in  upper  levels  of  the  circu- 
lation additional  cooling  is  caused  by  radiation, 
chiefly  from  the  clouds,  and  by  mixture  with  the 
colder  upper  air ;  hence  the  actual  cooling  of  the 
ascending  members  of  the  cyclonic  area  is  better 
represented  by  the  line,  BD.  Now,  in  order  that 
the  circulation  should  be  maintained  and  that  the 
air  which  has  ascended  in  cyclones  should  descend 
spontaneously  in  anticyclones,  it  must  be  assumed 
that  a  considerable  additional  cooling  takes  place 
during  the  passage  from  the  upper  part  of  the 
cyclonic  area  to  the  upper  part  of  the  anticyclonic 
area ;  this  being  indicated  by  the  line,  DE.  As  FIG.  70. 


222 


ELEMENTARY    METEOROLOGY. 


descent  in  the  anticyclone  begins,  warming  by  compression  soon  overcomes 
cooling  by  radiation,  and  for  the  greater  part  of  the  descent  the  increase  of 
temperature  follows  closely  the  adiabatic  rate,  FH ;  but  on  approaching  the 
ground,  departure  from  the  adiabatic  rate  will  be  made  in  accordance  with 
the  temperature  of  the  land  or  sea  over  which  the  descent  takes  place.  In 
winter  on  a  continent  the  change  of  temperature  in  the  lower  part  of  the 
circulation  would  be  indicated  by  GJ;  and  a  distinct  warming  and  moistening 
of  air  so  cold  and  dry  as  J  must  take  place  before  it  could  again  spontaneously 
enter  a  cyclonic  ascent.  It  is  therefore  manifest  that  the  essential  conditions 
of  the  spontaneous  convection  here  assumed  involve  on  the  whole  a  higher 
temperature  in  the  ascending  air,  as  ABD,  than  in  the  descending  air,  as  FGJ ; 
a  decided  cooling,  DEF,  in  the  upper  air ;  and  in  the  winter  season  a  cooling 
and  warming  again,  GJA,  in  the  lower  air. 

Fig.  71  represents  the  conditions  of  a  driven  convectional  circulation. 
The  lower  air  cools  at  the  rate,  AB,  at  first,  and  at  the  rate,  J5Z>,  after  it 
becomes  cloudy  j  the  departure  of  BD  from  BC  being  due  to  radiation  from  the 
lofty  clouds,  as  explained  in  the  previous  case.  A 
moderate  amount  of  cooling,  DE,  being  now  assumed 
as  occurring  during  the  passage  of  the  upper  air  from 
the  cyclonic  to  the  anticyclonic  region,  the  more 
active  anticyclonic  descent  causes  an  increase  of 
temperature  at  the  rate,  FH.  On  nearing  the  earth, 
especially  in  winter  and  over  land,  the  temperature 
curve  departs  from  the  adiabatic  line,  GH,  and 
becomes  GA.  The  looped  lines  here  drawn  indicate 
that  the  greater  part  of  the  cyclonic  air  is  cooler  than 
the  anticyclonic,  but  that  the  reverse  relation  obtains 
in  the  higher  regions.  The  altitude,  K,  at  which 
the  looped  lines  cross,  evidently  depends  chiefly  on 
the  amount  of  cooling  in  the  upper  air. 

In  choosing  between  the  theories  represented  by 
the  two  diagrams,  it  seems  that  the  latter,  which 
assumes  a  moderate  cooling  in  the  upper  air  and  a 
greater  cooling  in  the  lower  air,  is  nearer  to  the  conditions  of  our  winter 
atmosphere  than  the  first  diagram.  But  another  and  more  critical  test  has 
been  provided  by  Dr.  Hann's  studies  of  records  from  the  Alpine  mountain 
observatories. 

Let  it  be  supposed  that  a  mountain  observatory  stands  at  the  height,  M, 
and  that  it  is  frequently  visited  by  cyclones  and  anticyclones.  Its  mean 
temperature,  M,  in  any  month  will  differ  in  one  way  from  the  \\w.\\\  of  the 
temperatures  observed  in  cyclones ;  and  in  another  way  from  the  mean  of  the 
temperatures  observed  in  anticyclones ;  and  the  sign  of  these  differences  will 


FIG.  71. 


CYCLONIC    STORMS    AND    WINDS.  223 

give  decisive  indication  as  to  which  is  the  better  of  the  two  theories  under 
consideration. 

The  highest  of  the  Alpine  mountain  observatories  is  on  the  Sonnblick,  at 
an  altitude  of  10.155  feet.  Although  its  location  is  not  so  directly  in  the  path 
of  frequent  and  strong  cyclones  and  anticyclones  as  might  be  desired,  yet  its 
records  seem  to  leave  no  doubt  as  to  which  theory  they  support.  Dr.  Hann's 
recent  essays  on  this  subject  show  clearly  that  as  far  as  observations  at  high 
levels  are  concerned,  the  mass  of  air  in  anticyclones  is  from  six  to  ten  degrees 
higher  than  the  air  at  corresponding  altitudes  in  cyclones.1 

As  far  as  the  testimony  of  the  Alpine  stations  is  concerned,  it  must  therefore 
be  accepted  as  being  distinctly  in  favor  of  the  theory  that  regards  our  cyclones  as 
driven  eddies  in  the  general  circumpolar  winds ;  while  anticyclones  are  regions 
where  the  upper  air  is  forced  to  descend  against  its  will.  The  work  thus  done 
in  raising  and  pushing  down  great  masses  of  air  must  be  recognized  as  a 
considerable  resistance  which  the  upper  winds  have  to  overcome ;  and  without 
it,  they  might  flow  faster  than  they  do. 

It  should  be  carefully  noted  that  the  position  of  K,  Fig.  71,  is  variable ; 
that  with  greater  cooling  aloft,  it  stands  lower ;  and  that  the  lower  this  point 
stands,  the  more  important  is  the  spontaneous  convectional  action  of  the  upper 
part  of  the  entire  circulation ;  but  all  of  this  is  at  present  simply  a  matter  of 
speculation.  It  should  be  further  noted  that  whenever  condensation  takes  place 
in  a  cyclone,  or  evaporation  in  an  anticyclone,  the  work  to  be  done  in  driving 
these  disturbances  is  somewhat  diminished.  In  the  cyclone,  condensation 
liberates  latent  heat,  and  the  raised  air  is  then  maintained  at  a  higher 
temperature,  and  hence  in  a  more  expanded  condition  than  it  would  be 
otherwise ;  some  of  the  uplifting  is  then  done  by  the  liberated  energy,  received 
from  absorbed  insolation  long  before  and  hundreds  or  thousands  of  miles 
away.  It  is  probably  for  this  reason  that  the  winter  cyclones  that  traverse 
the  northern  United  States  generally  increase  so  greatly  in  energy  as  they 
approach  the  Atlantic  coast,  and  receive  an  Inflow  of  warmer  and  moister  air 
from  over  the  ocean.  Conversely,  when  occasional  clouds  are  evaporated  in 
the  upper  part  of  an  anticyclone,  its  warming  in  descent  is  retarded,  and  it  is 
more  easily  compressed  and  driven  down.  This,  however,  is  a  small  aid 
compared  to  that  given  by  cloud-making  and  rainfall  in  the  cyclones. 

Xo  test  by  means  of  mountain  observatories  has  yet  been  applied  to  tropical 
cyclones  ;  but  the  conditions  of  Fig.  70  may  be  reasonably  interpreted  as 
favoring  their  spontaneous  convectional  origin.  It  should  be  noticed  that  the 
initial  temperature  at  the  beginning  of  cyclonic  ascent  is  higher  in  the  first  of 

1  The  comparison  here  drawn  between  temperatures  at  the  same  altitude  in  cyclones  and 
anticyclones  should  be  more  strictly  drawn  between  altitudes  where  the  same  pressure  occurs 
in  the  two  ;  but  if  allowance  be  made  for  this,  the  excess  of  temperature  is  still  distinctly  in 
favor  of  the  anticyclone,  according  to  Dr.  Harm's  figures. 


224  ELEMENTARY    METEOROLOGY. 

the  two  figures ;  this  construction  having  been  adopted  in  order  to  apply  the 
diagram  to  tropical  examples.  The  cooling  during  ascent,  indicated  by  the  line, 
ABD,  Fig.  70,  is  consequently  slower  than  that  of  the  same  line  in  Fig.  71 ; 
because  the  first  example  represents  a  beginning  at  high  temperature  and 
humidity,  when  latent  heat  is  plentifully  set  free.  But  the  chief  difference 
between  the  two  figures  is  found  in  the  amount  of  cooling  assumed  in  the 
upper  air ;  and  it  is  manifest  that  the  greater  cooling  of  Fig.  70  is  not 
consistent  with  the  conditions  of  the  torrid  zone.  It  may  therefore  be  suggested 
that  in  tropical  cyclones  the  ascending  air  does  not  soon  return  to  sea  level, 
but  remains  for  a  considerable  time  at  a  great  height,  gradually  losing  its 
abnormally  high  temperature  there ;  and  in  confirmation  of  this,  the  absence 
of  distinct  anticyclones  in  the  torrid  zone  may  be  adduced.  The  increase  of 
temperature  indicated  by  FGA  must  therefore  be  taken  to  apply  to  air  masses 
settling  down  towards  sea  level  around  the  cyclonic  center,  but  not  supplied 
by  immediately  previous  ascent.  But  whatever  value  these  diagrams  possess, 
it  must  be  continually  borne  in  mind  that  they  are  for  the  greater  part 
speculative  ;  and  their  possible  but  imperfectly  proved  consequences  must 
be  clearly  separated  from  consequences  more  closely  based  on  observation. 

PROGRESSION  OF  CYCLONES. 

240.  Progression  of  cyclones.  The  cyclones  of  both  the  torrid  and 
temperate  zone  have  been  described  as  travelling  storms.  They  generally 
advance  along  well-defined  tracks,  peculiar  to  the  region  of  their  occurrence. 
Distinction  must  be  carefully  made  between  the  velocity  of  the  winds  around 
the  cyclonic  center  and  the  velocity  of  progression  of  the  entire  storm  from 
place  to  place  ;  there  is  no  essential  relation  between  the  two  quantities.  The 
violent  hurricanes  of  the  torrid  zone  move  slowly  along  their  tracks ;  the 
cyclones  of  the  temperate  latitudes  are  sometimes  violent  when  advancing 
slowly,  or  of  moderate  strength  when  advancing  rapidly  ;  but  the  reverse 
relation  is  also  observed.  No  general  connection  exists  between  the  two 
velocities. 

The  simplest  and  most  probable  explanation  of  the  movement  of  cyclones 
is  found  in  the  fact  that  they  are  disturbances  set  up  in  an  atmosphere  that  is 
already  in  motion.  This  is  plainly  the  case  with  extra-tropical  cyclones  ;  their 
movement  accords  so  well  with  the  average  direction  and  with  the  seasonal 
changes  of  the  general  winds  that  there  can  be  little  doubt  that  they  advance 
with  the  atmosphere  in  which  they  are  formed.  It  must,  however,  be 
recognized  that  it  is  difficult  to  conceive  of  a  cyclonic  whirl  maintaining  its 
action  in  an  atmosphere  whose  velocities  in  the  upper  and  lower  layers  differ 
so  greatly.  The  whirl  cannot  drift  bodily  ;  it  must  continually  be  reconstituted 
as  it  advances.  Cyclonic  trucks  have  been  shown  in  Fi^.  (\'2.  They  exhibit  a 
striking  agreement  with  the  course  of  the  general  winds  around  the  pole. 


CYCLONIC    STORMS    AND    WINDS. 


225 


They  move  faster  in  winter  than  in  summer,  as  should  be  expected  from  the 
winter  increase  in  the  velocity  of  the  circurnpolar  winds.  Cyclones  over  the 
eastern  United  States  move  nearly  twice  as  fast  as  over  western  Europe,  as 
should  follow  from  the  difference  in  the  velocity  of  the  lofty  cloud-bearing 
currents  in  the  two  regions  (second  Table  of  Sect.  212).  The  following  results 
have  been  determined  by  Loornis. 

AVERAGE  VELOCITY  OF  PROGRESSION  OF^CYCLONES. 

United  States      ........          .28.4  miles  per  hour. 

North  Atlantic  (middle  latitudes)  .     ,     .     .     18.0  «* 

Europe .  16.7  " 

West  Indies   ......     .     .     ,     ,     .     14.7  » 

Bay  of  Bengal  and  China  Sea    .....       8.5  " 


MONTH. 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

United  States     .... 
Europe           .          ... 

33.8 
17  4 

34.2 
18.0 

31.5 

17.5 

27.5 
16.2 

25.5 
14.7 

24.4 
15.8 

24.6 
14.2 

22.6 
14.0 

24.7 
17  3 

27.6 
19  0 

29.9 
18  6 

33.4 
17  9 

The  relation  of  the  tracks  of  tropical  cyclones  to  the  course  of  the 
general  winds  is  not  at  once  apparent ;  but  the  general  movement  of  these 
cyclones  around  the  western  border  of  their  oceans,  as  illustrated  in  Fig.  62, 
suggests  their  control  by  the  eddies  of  the  general  winds,  explained  in  Section 
157.  Even  in  the  doldrums,  it  must  be  remembered  that  only  the  lower  air  is 
calm.  At  greater  heights,  the  equatorial  overflow  turns  westward  and  towards 
the  pole  ;  hence  the  storms  forming  near  the  equator  always  recede  from  it. 
Their  movement  is  at  first  slow  and  obliquely  westward,  somewhat  accordant 
with  the  course  of  the  trade  winds  ;  but  on  passing  latitude  25°  or  30°  and 
entering  the  zone  of  the  westerly  winds,  they  turn  obliquely  to  the  east  and 
quicken  their  pace.  Cyclones  that  originate  in  the  eastern  part  of  an  equatorial 
ocean  move  like  the  others  obliquely  poleward  and  westward,  but  they  seldom 
succeed  in  escaping  from  the  torrid  zone  against  the  inflow  of  the  surface 
winds. 

241.  Effect  of  rainfall  on  progression.  An  additional  cause  has  been 
suggested  for  the  eastward  movement  of  extra-tropical  cyclones.  The  larger 
area  of  rainfall  occurs  on  their  eastern  side,  within  the  body  of  warmer  winds 
that  move  poleward.  Whatever  aid  is  given  to  the  action  of  a  cyclone  by  the 
liberation  of  latent  heat  is  therefore  unsymmetrically  placed  continually  to  the 
eastward  of  the  center,  instead  of  symmetrically  around  it,  as  in  tropical 
cyclones ;  and  as  far  as  this  is  an  effective  cause  of  motion,  it  tends  to 
transplant  the  storm  eastward ;  not  by  moving  it  along,  but  by  continually 
re-creating  it  to  the  east  of  its  previous  position. 


226  ELEMENTARY    METEOROLOGY. 

A  consequence  of  the  unsymmetrical  distribution  of  temperature  here 
referred  to  is  found  in  the  later  arrival  of  the  lowest  pressures  near  the  center 
of  our  cyclones  on  mountains  than  at  lowland  stations  near  by.  The  isobaric 
surfaces  are  held  further  apart  by  the  relatively  high  temperature  of  the  air  in 
front  of  the  storm ;  they  are  pressed  closer  together  in  the  rear  where  the  air 
is  colder.  As  seen  in  vertical  cross  section,  they  would  be  deformed  from  a 
symmetrical  to  an  unsymmetrical  arrangement.  The  line  l  joining  the  lowest 
points  in  the  successively  higher  isobaric  curves  therefore  leans  backwards, 
and  when  the  lowest  pressure  is  felt  at  sea  level,  the  pressure  is  still  falling  at 
adjacent  mountain  stations. 

242.  Effect  of  progression  on  the  velocity  and  direction  of  cyclonic  winds. 
The  progression  of  cyclones  is  generally  spoken  of  as  if  the  entire  storm 
moved  bodily  forward.  This  seems  to  be  true  in  large  measure  for  cyclones  at 
sea  ;  but  it  is  not  true  for  the  lower  winds  of  cyclones  on  land. 

The  bodily  advance  of  the  whole  storm  would  require  that  the  winds  on  the 
two  sides  of  the  storm  track  should  have  different  velocities,  as  felt  by  observers 
who  do  not  advance  with  the  storm.  The  winds  on  the  right  of  the  track  (in 
the  northern  hemisphere),  moving  around  the  center  in  the  direction  of  the 
storm's  advance,  would  be  increased  by  the  velocity  of  progression ;  while  the 
winds  on  the  left  side  of  the  track,  moving  with  same  velocity  around 
the  center,  would  be  decreased  by  the  velocity  of  progression.  Hence  the 
winds  on  the  two  sides  should,  under  this  supposition,  differ  in  average 
velocity  by  about  twice  the  velocity  of  the  storm's  progress.  At  the  same 
time,  the  inclination  of  the  winds  towards  the  center  should  be  increased  in 
the  rear  of  the  storm  and  diminished  in  the  front. 

Results  of  this  kind  have  been  found  in  the  case  of  several  tropical  cyclones 
at  sea ;  a  number  of  West  Indian  hurricanes  have  been  found  to  have  more 
violent  winds  on  the  right  than  on  the  left  of  their  tracks,  and  a  greater 
inclination  of  the  winds  in  the  rear  than  in  front;  here  the  velocity  of 
progression  is  small,  and  the  velocity  of  the  winds  is  high,  and  consequently 
the  difference  in  the  estimated  strength  of  the  winds  on  the  two  sides  is  not 
great.  A  much  clearer  illustration  of  this  relation  is  found  in  the  cyclones  of 
the  North  Atlantic  in  temperate  latitudes.  Westerly  gales  are  common  on  the 
south  of  the  cyclonic  centers,  but  easterly  gales  are  rare  on  the  north.  As  the 
general  velocity  of  progression  is  here  about  fifteen  miles  an  hour  eastward. 
the  winds  on  the  two  sides  of  the  track  might  have  velocities  of  thirty  and 
sixty  miles  an  hour  to  an  observer  who  did  not  advance  with  the  storm  center, 
although  the  winds  on  all  sides  of  tin-  storm  had  equal  velocities  of  forty-five 
miles  an  hour  around  the  center. 

JThis  line  is  sometimes  called  the  axis  of  the  storm.  The  term  is  misleading,  because  in 
the  case  of  whirling  on  an  axis,  it  is  always  implied  that  the  motion  takes  place  in  planes  a* 
right  angles  to  the  axis ;  and  this  is  not  the  case  here. 


CYCLONIC    STORMS    AND    WINDS.  227 

A  bodily  drifting  of  cyclones  is  also  apparent  at  moderate  altitudes  in  the 
atmosphere.  On  Mt.  Washington,  for  example,  where  regular  observations 
were  maintained  for  a  number  of  years  at  an  elevation  of  6,279  feet  by  our 
Signal  Service,  easterly  winds  on  the  northern  side  of  the  centers  of  low 
pressure  were  generally  faint,  but  the  westerly  winds  on  the  Southern  side 
often  attained  great  violence.  The  inclinations  also  showed  a  systematic 
variation.  At  still  greater  altitudes,  where  the  general  winds  move  very  fast, 
the  clouds  show  a  whirling  spiral  outflow  with  respect  to  a  rapidly-advancing 
cyclonic  center,  and  yet  all  its  parts  would  have  an  eastward  motion  with 
respect  to  the  earth's  surface,  as  appears  to  be  the  case  in  Fig.  66. 

The  case  is  quite  different  with  the  lower  cyclonic  winds  on  land.  In  our 
country,  the  average  eastward  progression  of  cyclones  is  twenty-five  or  thirty 
miles  an  hour.  If  the  storms  were  bodily  carried  over  the  earth,  the  winds 
on  the  two  sides  should  differ  by  fifty  or  sixty  miles  an  hour ;  that  is,  there 
might  be  a  calm  on  the  northern  side,  while  a  gale  of  sixty  miles  an  hour 
raged  on  the  southern.  This  is  by  no  means  the  fact.  The  winds  do  not 
differ  greatly  in  velocity  or  inclination  on  the  two  sides  of  our  storm  centers. 
Their  velocity  depends  much  more  on  the  value  of  the  local  gradients  than  on 
the  velocity  of  the  storm's  progression  or  on  their  position  in  the  storm. 

It  has  therefore  been  supposed  that  at  sea,  where  there  is  relatively  little 
friction,  the  advance  of  a  cyclonic  whirl  is  effected  by  a  forward  carriage  of 
the  whole  commotion ;  but  on  land,  where  the  surface  is  much  more  irregular, 
the  advance  of  the  whirl  as  a  whole  is  limited  to  its  middle  and  higher  parts, 
while  its  lower  winds  move  only  in  accordance  with  the  variations  of  pressure 
that  are  brought  on  them  from  above. 

243.   Veering  and  backing  of  winds  caused  by  passage  of  cyclones. 

Records  of  the  wind  at  stations  in  the  temperate  zone  have  long  shown  the 
occurrence  of  systematic  but  unperiodic  shifts  in  the  direction  of  the  wind 
along  with  changes  in  other  weather  elements. 

These  were  recognized  and  popularly  used  as  means  of  foretelling  weather 
changes  long  before  any  knowledge  had  been  gained  of  the  vorticular  whirling 
of  cyclonic  winds  or  of  the  progressive  motion  of  cyclonic  storms.  The 
alternation  of  northerly  and  southerly  winds  was  by  some  ascribed  to  the 
varying  success  in  a  struggle  between  polar  and  equatorial  currents ;  but  it 
may  be  now  confidently  asserted  that  this  is  impossible,  because  such  currents 
in  temperate  latitudes  must  flow  prevailingly  from  the  west,  with  relatively 
small  north  or  south  components  (Sect.  147). 

The  recognition  of  the  control  of  ordinary  weather  changes  by  cyclonic 
storms  of  greater  or  less  size  and  intensity  was  first  clearly  made  by  Redfield 
in  18.>4,  and  constitutes  an  important  advance  in  the  understanding  of  atmo- 
spheric processes.  Redfield's  generalization  was  fully  confirmed  some  thirty 


228 


ELEMENTARY    METEOROLOGY. 


years  later,  when  daily  weather  maps  were  prepared  in  different  countries, 
and  it  now  serves  as  the  most  important  principle  used  in  the  daily  official 
predictions  of  the  weather.  It  will  be  briefly  explained  here,  and  more  fully 
illustrated  in  Chapter  XIII. 

Fig.  72  is  an  ideal  diagram,  representing  the  distribution  of  the  various 
weather  elements  around  a  well-developed  center  of  low  pressure.  The 
concentric  oval  lines  are  isobars  for  every  two-tenths  of  an  inch;  the  curved 
broken  lines  are  isotherms  for  every  10°  F.  Imagine  the  entire  disturbance 


FIG.  72. 

to  advance  in  an  east-northeast  direction  at  a  rate  of  thirty  miles  an 
hour,  or  720  miles  a  day.  An  observer  at  any  station  over  which  it 
passed  would  stand  successively  under  different  parts  of  the  storm,  and  would 
experience  the  various  kinds  of  weather  that  it  brings  together.  If  his 
station  lay  on  the  track  of  the  storm  center,  his  sneei-ssm*  positions  within 
the  storm  area  would  fall  along  the  line,  ABC;  if  the  storm  center  passed  to 


CYCLONIC    STORMS    AND    WINDS.  229 

the  north,  his  station  would  be  found  on  the  line,  DEF ;  if  the  storm  passed 
to  the  south,  his  path  through  it  would  run  on  the  line,  GHJ.  In  the  case  of 
his  lying  on  the  track  of  the  center,  he  would  note  at  first  weak  southeasterly 
winds,  increasing  in  strength  and  shifting  somewhat  to  the  south,  with 
falling  pressure,  rising  temperature,  and  increasing  cloudiness  with  rain  or 
snow.  On  the  close  approach  of  the  center,  when  the  barometer  reaches  its 
lowest  reading,  the  winds  commonly  weaken  ;  and  shortly  afterwards  turn  more 
or  less  abruptly  to  the  northwest,  then  increasing  to  a  gale,  as  the  barometer 
rises  and  the  temperature  falls  ;  the  rain  soon  ceasing,  and  the  clouds  break- 
ing away  before  the  wind  takes  its  more  customary  direction  and  rajte. 

If  the  observer's  station  lie  south  of  the  track,  there  will  be  no  abrupt 
change  in  the  course  of  the  wind,  unless  the  form  of  the  storm  as  indicated  by 
its  isobaric  lines  is  very  unsyinmetrical ;  ordinarily  the  winds  begin  in  the 
south  or  southeast,  and  shift  with  considerable  regularity  through  the  south 
to  the  west  or  northwest ;  this  direction  of  change  being  called  veering.  If, 
on  the  other  hand,  the  observer  stands  north  of  the  track,  the  winds  will  begin 
in  the  east,  and  shift  through  the  northeast  and  north  to  the  northwest ;  this 
change  being  called  backing.  The  abnormal  direction  of  shift  implied  in  the 
term  "  backing "  is  simply  explained :  European  stations,  whence  these  terms 
have  come  to  us,  lie  generally  to  the  southeast  of  the  tracks  of  their  stronger 
Avinter  storms ;  hence  the  ordinary  shift  of  the  winds  is  from  the  east  through 
the  south  to  the  west,  or  "  with  the  sun,"  as  it  is  often  called ;  it  is  only  when 
an  exceptional  storm  center  passes  further  south  than  usual,  or  only  at  the 
distant  stations  in  northern  Europe,  that  the  winds  turn  "  against  the  sun " ; 
and  this  change  consequently  came  to  be  looked  on  as  the  reverse  of  the 
normal  order.  While  it  is  truly  of  unusual  occurrence  in  those  regions  where 
storm  centers  generally  pass  to  the  north,  it  is  manifestly  the  normal  order  of 
change  for  regions  whose  storms  habitually  pass  to  the  south.  As  our  cyclonic 
centers  generally  pass  over  the  Great  Lakes  and  down  the  St.  Lawrence 
valley,  observers  in  jbhe  United  States  are  accustomed  to  veering  winds ;  but 
observers  in  northern  Canada  may  normally  have  backing  winds. 

The  changes  of  wind  and  weather  thus  described  may  be  faintly  marked 
during  the  passage  of  cyclonic  areas  of  slight  barometric  depression  ;  but 
during  an  ordinary  season  an  attentive  observer  may  detect  twenty  or  thirty 
unmistakable  examples  of  cyclonic  shifts.  If  the  cyclonic  center  pass  far  to 
one  side  of  the  observer,  its  effects  are  hardly  noticed ;  but  if  its  passage  be 
nearly  central,  the  changes  in  its  wind,  temperature  and  sky  will  be  most 
emphatic.  If  the  center  advances  gradually,  the  changes  in  the  weather 
elements  will  be  slow  ;  if  the  center  progresses  rapidly,  the  changes  will  be 
quickly  run.  If  the  center  turn  to  the  north  or  south  of  the  ordinary  path,  the 
shifts  will  be  somewhat  abnormal  and  unexpected  ;  but  as  most  cyclones  follow 
a  tolerably*  regular  track  across  our  northern  states,  the  ordinary  sequence  of 


230  ELEMENTARY    METEOROLOGY. 

weather  changes  is  repeated  over  and  over  again  with  astonishing  regularity, 
the  intervals  between  the  generally  cloudy  and  wet  cyclonic  areas  being 
occupied  by  the  fair  and  generally  quiet  weather  of  anticyclonic  areas. 
Certain  of  the  winds  associated  with  the  cyclonic  inflow  are  marked  by  so 
many  distinct  features  that  they  deserve  special  names  and  descriptions, 
and  to  these  we  may  now  proceed. 

CYCLONIC  AND  ANTICYCLONIC  WINDS. 

244.  Frequence  of  cyclonic  winds  in  the  temperate  zone.    One  of  the  chief 
contrasts  between  the  conditions  of  the  torrid  and  temperate  zone  is  the  rarity 
of  cyclonic  disturbances  in  the  general  winds  of  the  former  and  their  frequent 
occurrence  in  the  latter.     In  the  torrid  zone,  day  after  day  and  month  after 
month  are  characterized  by  the  recurrence  of  similar  diurnal  changes  ;  on  land, 
warm  days  with  lively  convectional  breezes  following  the  course  of  the  general 
winds,  relatively  cool  nights,  and  only  occasional  interruptions  from  passing 
storms.     At  sea,  even  a  greater  regularity  of  changes  in  wind  and  temperature 
prevails. 

Such  constancy  is  unknown  in  the  temperate  zone,  where  there  is  a 
continual  change  from  one  kind  of  weather  to  another  in  irregular  periods 
generally  of  greater  length  than  a  day.  Nearly  all  of  these  changes  are 
the  product  of  passing  cyclonic  or  anticyclonic  disturbances.  They  may  be 
stronger  or  weaker ;  in  some  cases  having  winds  of  sufficient  activity  to 
deserve  the  name  of  storms,  in  other  cases  producing  only  gentle  breezes,  yet 
generally  associated  with  centers  of  low  or  of  high  pressure  of  greater  or  less 
distinctness,  with  the  accompanying  shifts  of  the  wind  and  with  areas  of  cloudy 
or  clear  sky.  The  rainfall  that  accompanies  the  cyclonic  areas,  or  that  comes 
from  the  smaller  storms  that  will  be  described  in  the  next  chapter,  constitutes 
our  chief  supply  of  precipitation ;  the  continual  alterations  from  fair  to  cloudy 
or  wet  weather  will  be  described  in  Chapter  XIII ;  but  before  considering 
these  subjects,  it  is  desirable  to  examine  certain  characteristic  winds  which 
accompany  the  stronger  cyclonic  and  anticyclonic  areas.  The  most  important 
of  these  are  the  sirocco,  the  cold  wave,  the  .bora,  the  foehn,  and  the  anti- 
cyclonic  calm. 

245.  The  warm  wave  or  sirocco.    The  wind  in  front  of  the  cyclonic  storms 
of  the  temperate  zone  blows  towards  the  pole.     It  advances  from  warmer  to 
colder  latitudes,  and  carries  with  it  a  relatively  high  temperature  characteristic, 
of  its  source.     In  the  eastern  part  of  the  United  States  this  wind  comes  from 
the  Gulf  of  Mexico  or  from  the  warm  waters  of  the  Gulf  Stream ;   it  brings 
unseasonable  and  oppressive  heat  to  the  central   and  northern   States,  and 
causes  a  rapid  rise  of  temperature  whether  arriving  by  day  or  night.     Curves 


CYCLONIC    STOKMS    AND    WINDS. 


231 


e  and  /,  Fig.  10,  are  examples  of  temperature  changes  under  warm  cyclonic 
winds  in  Xew  England.  A  wind  of  this  kind  is  generally  damp  and  cloudy  with 
rain  in  winter,  from  having  cooled  on  its  way  until  its  upper  portion  at  least 
has  been  chilled  below  the  dew-point.  Observations  on  Mt.  Washington  give 
many  examples  in  front  of  approaching  cyclonic  centers,  where  the  temperature 
in  winter  a  mile  above  sea  level  is  actually  higher  than  that  on  the  cold 
lowlands  over  which  the  wind  blows ;  this  is  because  the  wind  moves  faster 
aloft  and  therefore  comes  more  quickly  from  a  farther  source ;  it  thus  starts 
with  a  high  temperature  and  soon  becoming  cloudy  retains  much  of  its  heat 
on  the  way;  while  the  wind  that  blows  from  the 
south  closer  to  the  earth's  surface  comes  slowly 
from  a  less  distance  and  is  much  more  cooled  in 
its  northern  progress.  The  vertical  temperature 
gradient  at  such  a  time  may  be  represented 
roughly  in  Fig.  73 ; 1  A  B  being  the  normal 
value  for  the  place  and  season,  and  CDEF 
being  the  temporary  value  during  the  blowing 
of  the  southerly  wind.  The  greatest  increase 
of  temperature  is  at  some  considerable  altitude, 
as  at  E\  and  there  may  be  thus  produced  an 
inversion  of  temperature  for  a  certain  distance 
above  the  earth's  surface,  as  CD.  Inversions 
of  this  kind,  caused  by  cyclonic  importation  of 
little  cooled  air  aloft  and  comparatively  inde-  - 
pendent  of  diurnal  changes,  should  be  compared 
with  the  inversions  at  night  described  in  Section 
43,  produced  by  the  local  cooling  of  the  quiet  lower  air  by  radiation  and  con- 
duction. As  the  reversed  gradient,  CD,  indicates  great  stability  in  the  lower 
air,  winds  of  this  class  possess  relatively  constant  strength  night  and  day,  and 
they  are  remarkably  free  from  the  day-time  flurries  and  flaws  of  our  northwest 
winds.  The  occasional  silver  thaws  and  ice  storms  of  our  winters  (Sect.  287) 
depend  on  inversions  of  this  kind  under  easterly  winds ;  the  temperature  at  D 
being  a  little  above  freezing,  while  at  C  it  is  below  freezing. 

In  the  spring  and  summer,  the  southerly  or  southwesterly  wind  in  front 
of  cyclones  in  this  country  may  be  dry  as  well  as  warm,  because  the  lands  over 
which  it  then  blows  already  possess  a  relatively  high  temperature  and  the 
strength  and  length  of  insolation  in  the  higher  latitudes  do  not  permit  the 
easy  cooling  of  the  wind.  Inversions  of  temperature  do  not  then  prevail. 
The  enervating  days  in  the  warm  spells  of  April  and  May  are  found  under 

1  All  the  figures  of  vertical  temperature  gradients  in  this  chapter  are  largely  hypothetical  : 
they  represent  graphically  certain  probable  conditions  that  cannot  easily  be  described  in 
words. 


FIG.  73. 


232  ELEMENTARY    METEOROLOGY. 

such  conditions  (see  curve  a,  Fig.  10) ;  the  prolonged  spells  of  intense  summer 
heat  are  caused  in  the  same  way.  The  "  hot  winds  "  of  Kansas  and  Texas 
seem  to  be  intensified  examples  of  this  kind  of  wind.  At  all  seasons,  it  is 
generally  in  the  presence  of  these  southerly  cyclonic  winds  that  the  highest 
temperatures  of  our  months  are  found. 

It  is  manifest  that  winds  of  this  kind  must  occur  wherever  the  spiral 
cyclonic  inflow  brings  warm  winds  over  a  cool  region.  In  western  Europe, 
although  the  southerly  or  southwesterly  wind  is  warm,  it  does  not  produce  the 
strong  rise  of  temperature  that  accompanies  it  in  the  eastern  United  States, 
because  of  the  much  slower  change  of  mean  temperature  with  latitude  there 
than  with  us  (Sect.  82).  Along  the  northern  shores  of  the  Mediterranean, 
however,  when  cyclonic  storms  pass  near  by,  a  warm  wind  blows  northward 
in  front  of  them  from  Africa.  It  is  sometimes  parching  hot  and  dry,  bearing 
dust  from  the  desert  and  seriously  injuring  vegetation  ;  its  heat  being  felt 
almost  as  severely  by  night  as  by  day.  Such  is  the  sirocco  of  southern  Italy 
and  Greece,  illustrated  in  Fig.  81  ;  but  further  north,  after  the  wind  has 
blown  over  a  greater  breadth  of  water  surface,  gaining  vapor  on  the  way  and 
at  the  same  time  cooling  somewhat  on  its  northward  journey  towards  the 
cyclonic  center,  it  becomes  moist  and  cloudy,  its  high  temperature  then  making 
the  air  extremely  sultry  and  oppressive.  In  Spain,  the  dry  sirocco  is  called 
the  leveche  ; l  on  the  Madeira  islands  it  is  called  the  leste.  The  harmattan, 
a  hot,  dusty  east  wind  on  the  west  coast  of  the  Sahara,  may  be  of  similar 
cyclonic  origin.2  In  Egypt,  the  representative  of  the  sirocco  is  known  as  the 
khamsin,  from  its  relatively  frequent  occurrence  during  a'  period  of  fifty  days 
in  early  spring,  when  the  southward  retreat  of  the  tropical  belt  of  high 
pressures  allows  cyclonic  storms  to  extend  their  influence  over  northern  Africa. 
The  association  of  these  various  winds  with  cyclonic  storms  is  not  demonstrated 
in  all  cases,  but  it  appears  to  hold  true  as  far  as  they  have  been  studied. 
Hence  the  advisability  of  recognizing  them  as  belonging  to  a  distinct  class  of 
atmospheric  phenomena,  deserving  a  particular  name.  The  Italian  name, 
sirocco,  might  be  adopted  in  any  part  of  the  world,  when  the  peculiar  features 
of  the  wind  are  well  developed.  It  is  not  intended  that  every  light  wind  in 
front  of  a  cyclonic  center  should  be  called  a  sirocco  ;  but  when  the  wind  is 
active  and  its  temperature  is  decidedly  above  the  normal,  this  name  may  be 
properly  given  to  it. 

In  the  southern  hemisphere,  winds  of  the  sirocco  class  come  from  the 
north.  Such  are  the  "  brickfielders "  or  hot  north  winds  of  southern 
Australia,  and  the  zonda  of  the  Argentine  pampas. 

1  The  solano  of  the  east  coast  of  Spain  is  a  cloudy,  rain-bringing  east  wind ;  probably  a 
cyclonic  indraft,  but  not  to  be  confused  with  tin  lmt  ami  dusty  leveche. 

0  The  harmattan  of  the  Gulf  of  Guinea  is  a  cool,  dry,  northerly  wind  of  the  winter 
•eason,  like  an  intensified  trade  wind. 


CYCLONIC    STORMS   AND    WINDS.  233 

The  "  hot  winds  "  of  Texas  and  Kansas,  above  referred  to,  seem  to  possess 
certain  special  features  in  addition  to  those  of  the  normal  sirocco.  The 
greatest  heat,  105  or  more  degrees,  is  felt  in  narrow  currents,  ranging  from  100 
feet  to  half  a  mile  or  more  in  width,  with  intermediate  belts  of  considerable 
breadth  at  less  insufferable  temperatures.  The  intense  heat  and  extreme 
dryness  of  these  winds  make  them  very  injurious  to  all  crops.  Their  heat 
does  not  seem  to  be  due  simply  to  importation  by  a  horizontal  flow,  and  it  has 
therefore  been  suggested  that  they  are  supplied  by  descending  currents,  warmed 
adiabatically.  The  apparent  difficulty  of  this  suggestion  lies  in  the  excessive 
temperature  that  the  currents  gain  at  the  level  of  the  ground,  in  virtue  of 
which  they  should  be  lighter  and  not  heavier  than  the  lower  air  into  which 
they  are  assumed  to  descend :  but  as  descent  from  a  great  height  must  be 
inferred  in  order  to  produce  their  excessive  temperature,  a  descent  by 
momentum  below  the  level  of  equilibrium  may  also  be  inferred  ;  and  thus  the 
local  and  temporary  quality  of  the  hottest  currents  might  be  explained.  Hot 
winds  of  a  similar  quality  occur  on  the  plains  of  India  in  the  early  summer ; 
and  it  is  possible  that  certain  of  the  suffocating  simooms  of  the  Arabian  and 
African  deserts,  excessively  hot  but  free  from  dust,  should  be  explained  in 
this  way ;  while  the  simooms  that  bear  clouds  of  sand  and  dust  should  be 
associated  with  thunder-squalls  (Sect.  255). 

246.  The  cold  wave.  The  warm  sirocco  in  front  of  a  cyclonic  storm  is  in 
distinct  contrast  with  the  cool  or  cold  equatorward  wind  in  the  rear.  In 
summer,  the  latter  wind  possesses  only  moderate  strength,  producing  an 
agreeable  cooling  after  the  excessive  heat  of  the  sirocco  that  it  drives  away ; 
it  may  then  be  called  a  cool  wave  ;  clear,  dry  and  refreshing,  after  the  sultry 
and  oppressive  air  that  preceded  it.  In  winter,  it  may  be  a  strong  wind, 
coming  quickly  and  causing  a  rapid  fall  of  temperature.  This  fall  is  called  a 
cold  wave  by  our  Weather  Bureau,1  and  the  term  may  be  applied  to  the  wind 
that  brings  it  and  extended  to  other  winds  of  the  same  kind. 

The  cold  wave  is  remarkably  well  developed  in  the  winter  storms  of  the 
central  and  eastern  parts  of  our  country.  When  supplied  by  an  area  of  strong 
high  pressure  in  the  northwest,  it  sweeps  down  from  the  cold  plains  of  farther 
( 1anada  and  brings  with  it  the  low  temperature  of  that  bleak  region.  Its 
movement  obliquely  towards  the  cyclonic  center  that  it  follows  is  accelerated 
by  the  winter  high  pressure  characteristic  of  its  source  in  the  continental 
center  (see  foot-note,  page  92),  and  it  is  nowhere  impeded  by  transverse 
mountain  ranges.  Near  the  track  of  a  cyclonic  storm,  the  cold  wave  arrives 
suddenly  and  in  almost  fully-developed  strength,  displacing  the  antecedent 

1  Technically,  a  "cold  wave"  is  a  fall  of  temperature  lower  than  32°  in  the  northwest, 
or  lower  than  40°  in  the  south,  with  a  change  of  at  least  20°  in  twenty-four  hours,  and 
without  regard  to  the  velocity  or  direction  of  the  wind. 


234 


ELEMENTARY    METEOROLOGY. 


sirocco  in  a  few  hours  and  causing  an  abrupt  fall  of  temperature,  as  in  Fig. 
lOc ;  and  then  gradually  falling  away,  being  in  this  unlike  the  sirocco,  which 
begins  gently  and  only  gradually  acquires  its  full  development.  All  the 
Mississippi  basin  and  the  Atlantic  states  as  far  south  as  Florida  may  feel  the 
blast  of  the  cold  wave  as  it  comes  sweeping  down  from  the  northwest ;  a  clear. 


FIG.  74.  JANUARY  9. 


FIG.  76.  JAM  ARY  8. 


FIG.  7H.  JAM  AH v  10. 


FIG.  75.  JANUARY  7. 


Fro.  77.  JANUARY  9. 


FIG.  79.  JANLAIM  '.'. 


cold,  dry  wind,  freezing  the  ground  over  which  it  advances,  and  thereby 
warming  somewhat  in  its  own  lower  layers.  It  may  cause  a  continuous  fall  of 
temperature  during  noon-day,  as  is  illustrated  in  Fig.  lOrf. 

A  very  strong  cold  wave  occurred  early  in  January,  1886,  as  illustrated  in 
the  accompanying  figures,  74-79.     The  cyclone  behind  which  the  cold  winds 


CYCLONIC    STORMS    AM)    WINDS. 


235 


were  drawn  down  from  their  source  in  the  far  northwest,  lay  centrally  on  the 
coast  of  Texas  on  the  morning  of  January  7  ;  advanced  to  southern  Alabama 
on  January  8,  and  to  Xew  Jersey  on  January  9,  when  its  nearly  circular 
isobars  were  remarkably  well  developed,  as  in  Fig.  74.  On  the  morning  of 
January  10,  the  center  lay  in  the  Gulf  of  St.  Lawrence.  Figs.  75  to  78  indi- 
cate the  spreading  of  the  cold  northerly  air  over  the  country  on  the  four  dates 
above  named ;  the  white  space  including  temperatures  between  zero  and 
freezing;  while  the  lined  shadings  represent  temperatures  lower  than  — 30° 
in  the  northwest  and  over  -f-  50°  in  the  south.  The  cold  air  that  lay  in  the 
northwest  on  January  7  had  reached  Texas  the  next  morning,  when  the 
isotherm  of  zero  ran  nearly  north  and  south  in  the  middle  Mississippi  valley. 
On  the  two  following  days,  the  zero  winds  advanced  up  the  Ohio  valley,  and 
carried  temperatures  below  freezing  even  into  Florida,  where  the  orange 
groves  were  seriously  injured.  The  retarded  advance  of  the  cold  around  the 
Great  Lakes  was  in  part  due  to  their  conservative  influence,  but  in  part  also 
to  the  derivation  of  their  winds  more  from  the  northeast  than  from  the  north- 
west. Fig.  79  exhibits  the  normal  temperatures  for  January  in  oblique  dotted 
lines  ;  its  shaded  areas  show  the  negative  departures  from  the  normals  on  the 
morning  of  January  9 ;  amounting  to  40°  in  the  central  Mississippi  valley. 

The  probable  value  of  the  vertical  temperature  gradient  in  a  cold  wave 
may  be  represented  in  Fig.  80  ;  the  greatest  cooling  from  the  normal  being  at 
an  elevation,  Z>,  of  half  a  mile  or  more,  where  the  winds  are  strongest.  The 
departures  of  the  gradient  lines  so  greatly  from 
the  mean  value  in  the  cold  wave  and  in  the 
sirocco  should  be  compared  with  the  smaller 
departures  illustrated  in  Figure  3,  page  27. 
The  latter  result  from  changes  in  the  tempera- 
ture of  relatively  quiet  air  by  the  slow  processes 
of  absorption  and  radiation ;  the  former  result 
from  the  active  cyclonic  importation  of  new 
bodies  of  air,  which  bring  with  them  the  tem- 
peratures characteristic  of  their  source,  thus 
strongly  affecting  the  atmosphere  up  to  heights 
of  more  than  a  mile.  Variations  of  absolute 
humidity  are  produced  in  the  same  way.  Non- 
periodic  changes  of  temperature  and  humidity 
in  the  higher  layers  of  the  atmosphere  are 
therefore  to  be  regarded  as  chiefly  caused  by 
the  changes  of  cyclonic  winds. 

Near   the    earth's   surface,   the   air   of   the 
cold  wave  generally  becomes  warmer  as  it  advances,  and  its  gradient  curve 
may  b*e  represented  by  dc  or  dc'.     This  is  particularly  the  case  in  the  day-time 


236  ELEMENTARY  METEOROLOGY. 

and  in  the  early  spring  when  there  is  no  snow  on  the  ground.  The  lower 
layers  of  the  wind  may  thus  become  unstable,  and  hence  there  is  at  such  times 
a  strong  variation  in  velocity  between  day  and  night ;  the  wind  of  day-time  is 
continually  hastened  by  rushing  flaws  and  flurries,  as  its  currents  roll  over 
and  over  in  their  convectional  turning,  while  the  lower  air  at  night  is 
comparatively  quiet.  But  when  the  cold  wave  blows  at  night  over  a  snow- 
covered  region,  the  temperature  of  the  air  near  the  earth's  surface  may 
decrease  as  it  advances ;  the  vertical  temperature  gradient  may  then  be 
reversed  in  the  lower  air. 

When  blowing  at  high  velocities  and  bearing  a  blinding  cloud  of  snow  with 
the  temperature  below  freezing,  the  cold  wave  is  called  a  blizzard.  The 
norther  of  Texas  and  the  Gulf  of  Mexico  includes  both  the  cold  wave  of 
winter  and  the  cool  wave  of  summer:  this  wind  is  frequently  developed 
in  the  autumn  on  the  western  side  of  a  tropical  cyclone  as  it  advances  slowly 
along  the  recurving  part  of  its  path  south  of  Louisiana.  When  following  a 
cyclonic  center  in  winter,  the  norther  may  cause  a  fall  of  30°  in  an  hour,  or  of 
50°  in  two  hours. 

The  winter  cold  wave  of  Europe  is  much, less  pronounced  than  with  us, 
and  comes  from  the  northeast  instead  of  from  the  northwest.  The  mild 
waters  of  the  North  Atlantic  lie  northwest  of  Europe,  hence  no  strong  fall  of 
temperature  is  brought  by  an  inflow  of  winter  winds  from  that  direction  over 
Great  Britain  and  France  in  the  rear  of  a  cyclonic  storm  that  passes  eastward 
over  the  North  Sea  toward  Russia ;  but  when  the  storm  center  moves  from  the 
Atlantic  across  southern  France  towards  Italy,  and  at  the  same  time  an  anti- 
cyclone lies  over  northern  Russia,  then  western  and  central  Europe  experience 
a  cold  northeast  wind,  akin  to  our  cold  waves.  The  wind  on  the  north  of  the 
cyclone  moves  southwestward  from  the  cold  northern  continental  area  and 
floods  Germany  and  France  and  even  Great  Britain  with  air  of  unusually  low 
temperature.  The  severe  frosts  of  the  winter  of  1890-91  in  Great  Britain 
accompanied  a  period  of  frequent  northeast  winds  of  this  character. 

If  a  cyclonic  center  passes  far  enough  south  to  draw  the  cold  air  after  it 
from  the  low  plateau  of  central  France,  the  wind  is  called  the  mistral  as  it 
flows  down  the  valley  of  the  Rhone  to  the  Mediterranean ;  the  name  being 
derived  from  magistral  meaning  master.  When  France  is  snow  covered  under 
a  clear  sky,  the  air  on  the  plateau  becomes  cold  from  local  radiation,  and  the 
gentle  slope  of  the  country  towards  the  Mediterranean  aids  the  baric  gradients 
in  hurrying  the  aerial  drainage  and  intensifying  the  wind  ;  thus  repeating  on 
a  smaller  scale  the  conditions  of  our  western  plains.  Marseilles  may  have  a 
cold  mistral  while  southern  Italy  suffers  under  an  oppressive  sirocco;  as 
illustrated  in  the  map  of  a  cyclone,  Fig.  81,  which  passed  northeast  across 
Italy  on  February  25,  1879 ;  the  value  of  the  isobars  being  in  millimeters,  the 


CYCLONIC    STORMS    AND    WINDS. 


237 


temperature  in  Fahrenheit  degrees  :  the  heavy  wind  arrows  indicating  stations 
of  observation,  while  the  fainter  arrows  are  added  to  generalize  the  course 
of  the  wind  around  the  center  of  low  pressure.  The  distortion  of  the 
isotherms  by  the  winds  is  perfectly  apparent. 

Central  Europe  never  feels  the  excessive  cold  that  is  produced  by  the  cold 
waves  of  the  upper  Mississippi  valley,  where  temperatures  of  twenty  or  thirty 
degrees  below  zero  are  recorded  ;  but  the  more  severe  winter  cold  of  Europe  is 
generally  experienced  under  such  conditions  as  have  been  just  described. 


K 


t 
s<-'  /  /     M    ' 


FIG.  81. 

The  name,  cold  wave,  is  not  employed  there,  although  it  is  perfectly  appli- 
cable. Further  east,  in  Russia  and  Siberia,  where  the  continental  extension 
allows  a  more  severe  winter,  the  colder  cyclonic  wind  is  more  like  our  cold 
wave  ;  when  blowing  violently  and  raising  a  cloud  of  fine  dry  snow,  it  is  called 
a  buran  or  purga,  corresponding  to  the  blizzard  with  us. 

The  southern  hemisphere  has  cool  waves  from  the  south  in  the  rear  of  its 
cyclonic  storms ;  but  in  the  absence  of  large  land  areas  in  high  latitudes,  the 
fall  of  temperature  is  never  as  violent  as  with  us ;  no  strong  cold  waves  occur 
there.  The  wind  of  this  kind  in  the  Argentine  Republic  is  called  the  pampero. 
The  "  southerly  burster  "  of  New  Zealand  also  seems  to  belong  here. 

247.  The  bora.  It  occasionally  happens  that  the  cold  wind  drawn  down 
from  high  plateaus  towards  a  cyclonic  center  takes  on  especial  features  which 
entitle  it  to  a  separate  description.  The  strong  radiation  from  an  elevated 
plateau  during  short  winter  days  of  weak  sunshine  and  long  clear  winter 


238  ELEMENTARY    METEOROLOGY. 

nights  reduces  the  air  that  lies  upon  it  to  an  abnormally  low  temperature.  If 
the  vertical  temperature  gradient  over  the  surrounding  lowlands  is  AB,  Fig. 
8.?,  it  may  be  CD  over  the  plateau.  Then  if  the  air  flows  off  of  the  plateau 
and  descends  rapidly  to  the  surrounding  lower  ground,  it  is  warmed  by 
compression  at  the  rate  CE  during  descent  towards  sea  level,  and  when  it 

arrives  on  the  adjacent  lowlands  it  still  has  an 
unusually  low  temperature,  E.  Under  ordinary 
conditions  the  drainage  of  the  plateau  is  relatively 
slow,  and  its  descent  is  not  strong  enough  or  in 
large  enough  volume  to  be  particularly  noticeable  ; 
but  if  an  advancing  cyclone  brings  pressure  gradi- 
ents over  the  plateau  area  in  such  a  way  as  to  drive 
the  air  rapidly  from  it,  it  flows  down  over  the 
adjacent  lowlands  in  large  volume  with  great 
velocity;  being  induced  to  move,  not  only  by  the 
cyclonic  gradients,  but  also  by  its  own  instability 
or  top-heaviness.  It  is  then  felt  as  an  icy  blast, 
generally  producing  boisterous  squalls,  sometimes 
with  snow  flurries  ;  the  latter  probably  due  to  some 
reaction  between  the  descending  air  and  that  whicli 
it  displaces. 

Winds  of  this  kind  are  not  of  common  occurrence. 
They  are  recognized  at  the  head  of  the  Adriatic 
sea,  where  the  name,  bora,  is  a  survival  of  the  classical  Boreas,  the  north 
wind.  Here  the  bora  comes  from  the  Istrian  and  Dalmatian  highlands  on 
the  northeast.  The  mistral  has  already  been  described  as  owing  part  of  its 
cold  and  violence  to  its  descent  from  the  low  plateau  of  central  France ;  but 
in  that  case,  the  descent  is  so  gradual  as  not  to  warrant  its  being  classified 
here.  Bora  winds  should  be  looked  for  in  our  western  plateau  regions,  where 
pronounced  examples  of  their  occurrence  may  be  expected.  Their  recognition 
will  constitute  one  of  the  advances  that  should  be  made  in  physical  meteor- 
ology by  local  observers. 

In  order  to  perceive  the  curious  contrast  between  winds  of  the  bora  type 
and  those  of  the  class  next  described,  it  must  be  borne  in  mind  the  bora 
requires  an  extended  area  of  elevated  country,  where  the  air  may  be  abnor- 
mally cooled,  and  whence  it  is  hastily  withdrawn  to  lower  levels  by  cyclonic  aid. 

248.  The  foehn  or  chinook.  One  of  the  most  peculiar  members  of  the 
class  of  cyclonic  winds  is  found  where  the  indraft  is  required  to  pass  down 
from  or  over  a  mountain  range  in  its  course  towards  the  cyclonic  cente-, 
Winds  of  this  kind  often  have  a  strong  development  in  the  northern  valleys  of 
the  Alps,  where  their  remarkable  heat  and  dryness  were  first  noted  and 


CYCLONIC    STORMS   AND   WINDS. 


239 


investigated ;  the  local  name,  foehn,  there  employed,  is  now  extended  to  other 
regions  as  well.  The  occurrence  of  the  Alpine  foehn  may  be  described  as 
follows  :  —  When  a  cyclonic  storm  advances  from  the  Atlantic  over  central  or 
northern  Germany,  the  air  in  front  and  to  the  south  of  it  is  successively 
drawn  in  obliquely  towards  the  center,  first  from  middle  Germany  and 
northern  Bavaria,  then  from  the  sloping  plateau  at  the  northern  base  of  the 
A.lps,  next  from  the  valleys  among  the  mountains  out  of  which  the  air  flows 
to  take  the  place  of  that  which  has  moved  away  from  the  piedmont  plateau ; 
later,  by  the  still  further  backward  propagation  of  the  disturbance,  even  the 
air  from  the  mountain  tops  is  drawn  down  into  the  valleys,  and  finally  a 
supply  of  air  for  the  mountain  tops  is  derived  from  the  further  side  of  the 
range,  either  at  the  average  height  of  the  crest  line  or  from  the  lowlands. 
The  peculiarity  of  this  wind  as  it  descends  into  the  northern  valleys  depends 
on  the  changes  of  temperature  produced  in  consequence  of  its  motion  having  a 
vertical  component ;  while  the  temperatures  characterizing  the  sirocco  and  the 
cold  wave  depend  essentially  on  their  horizontal  motion. 

When  the  upper  air  is  drawn  quickly  down  from  the  Alpine  crests  into  the 
valleys  below,  it  is  heated  by  compression  at  the  normal  adiabatic  rate,  and 
from  having  been  a  cold  wind  on  the  mountain  tops,  it  reaches  the  valleys 
abnormally  warm  and  dry.  This  is  particularly  the  case  in  winter ;  the  air  in 
the  Swiss  valleys  before  the  approach  of  a  cyclonic  storm  becomes  especially 
cold  and  damp  or  foggy  by  the  accumulation  of  cooled  air  that  creeps  down  the 
mountain  sides  and  settles  in  the  depressions ;  while  at  the  same  time  the  air 
bathing  the  peaks  is  comparatively  dry,  and  is 
relatively  little  cooled,  and  departs  less  from  the 
mean  of  the  year.  Under  such  conditions,  the 
vertical  temperature  gradient  CD  or  C'D,  Fig.  83, 
would  be  weaker  than  its  annual  value,  AB.  Now 
if  the  cold  bottom  air  is  rapidly  withdrawn  to 
supply  a  cyclonic  indraft,  the  air  from  the  level  of 
the  mountain  tops  must  as  rapidly  descend  into 
the  valleys.  As  it  descends  it  warms  rapidly, 
almost  at  the  adiabatic  rate,  DJ,  while  its  dew- 
point  rises  but  little  faster  than  at  the  rate  ed,  dm* 
to  compression  (see  page  163);  and  on  its  arrival  at 
the  valley  bottom,  with  temperature  E  and  dew- 
point  F,  it  is  much  higher  in  temperature  and 
much  lower  in  humidity  than  was  the  air  which 
occupied  the  valley  before.  In  summer,  these  Flo  ^ 

changes  are  not  so  marked,  because  then  the  air  in 

the  valley  may  be  unduly  warm,  and  when  the  upper  air  is  drawn  down  to 
take  its  place,  no  significant  change  of  temperature  will  be  introduced, 


240 


ELEMENTARY    METEOROLOGY. 


although  there  may  still  be  a  decrease  of  humidity;  but  in  winter,  botli  the 
heat  and  the  dry  ness  of  the  foehn  are  remarkable.    The  snow  on  the  mountain 


FIG.  84. 

slopes  disappears  under  its  heated  breath;  hence  the  name  Schneefresser, 
or  snow-eater,  sometimes  locally  given  to  it.  Extensive  fires  among  the 
wooden  houses  of  the  Swiss  villages  have  happened  at  such  times,  the  last  one 
-  of  the  kind  being  that  which  destroyed 

Meiringen  in  the  winter  of  1891-92. 
The  peculiar  heat  and  dryness  of  the 
foehn  are  soon  lost  as  it  advances 
across  the  Piedmont  plateau,  cooling 
and  absorbing  vapor  on  its  way. 

After  the  initiation  of  the  foehn  as 
thus  described,  it  may  be  continued  by 
a  further  supply  of  air  coming  from  the 
plains  of  northern  Italy,  and  then  an 
additional  cause  for  the  heat  and  dry- 
ness  of  the  wind  is  introduced.  When 
the  air  is  drawn  away  from  the  summit 
of  the  Alps,  other  air  rises  from  the 
Italian  lowlands,  passes  over  the  range, 
and  descends  on  the  leeward  slopes, 
Fig.  84.  Before  ascent,  the  temperature 
of  the  air  on  the  Italian  plains  may 
be  represented  by  B,  Fig.  85,  while 
the  temperature  is  A  in  the  northern 
valleys.  During  the  ascent  of  the  Ital- 
ian air,  its  temperature  falls,  its  dew- 
point  is  reached,  the  whole  mass  becomes 
cloudy,  and  rain  or  snow  falls  from  the 


v« 

\                                        10,000- 

\ 

+ 

\ 

\ 

X    - 

*• 

1     u 

1     \> 

\\ 

1      \N\ 

* 

\  \\ 

\  \\ 

ft      \\  • 

4- 

\  \\ 

)•       \  \v 

'•                         V 

0.      V 

\ 

i  >                    \ 

+  N\ 

D> 

i     \\ 

\     \\ 

:  i 

\     \  \ 
\      \  \         i 

\  i 

Ac  \\ 

i  \ 

\  v« 

\ 

FIG.  85. 


clouds  on  the  Italian  slope.  In  the  first  part  of  the  ascent,  before  the 
level  is  reached,  the  temperature  of  the  ascending  air  decreases  at  the 
rate  2?<7;1  but  when  clouds  are  formed  at  the  height  C  and  above,  further 

1  It  must  be  borne  in  mind  that  the  horizontal  scale  of  Fig.  85  indicates  temperature  only, 
and  hence  that  its  oblique  lines  represent  only  rates  of  cooling  with  ascent  or  descent:  they 
have  nothing  to  do  with  the  inclined  path  of  the  air  over  the  mountains,  illustrated  in  Fig.  84, 
but  only  with  the  effect  of  the  vertical  components  of  motion  during  the  passage. 


CYCLONIC    STORMS    AND    WINDS.  241 

cooling  is  retarded  by  the  liberation  of  latent  heat  from  the  condensing  vapor, 
the  rate  of  cooling  then  being  CE,  a  brief  ascent  without  cooling  occurring  at 
the  temperature  of  freezing,  Z>  (Sect.  198).  The  temperature  to  which  the  air 
is  reduced  on  reaching  the  level  of  the  mountain  crests  is  therefore  not  so  low 
as  it  would  have  been  if  it  had  risen  to  that  height  without  becoming  cloudy. 

As  the  wind  flows  down  from  the  peaks  and  passes  of  the  Alps,  the  clouds 
that  it  carries  along  are  soon  dissolved  by  the  increase  of  temperature  pro- 
duced as  the  air  descends  to  the  northern  valleys;  for  a  great  part  of  the 
vapor  has  been  taken  from  the  wind  to  fall  as  rain  or  snow  on  the  Italian 
slope ;  the  remaining  cloud  mass  stands  on  the  mountain  crests,  and  is  locally 
known  as  the  foehn  wall.  As  long  as  any  cloud  remains  to  be  dissolved,  the 
increase  of  temperature  in  the  descending  air  goes  on  at  the  slow  rate,  EF, 
just  as  the  decrease  of  temperature  was  slow  during  the  cloudy  ascent;  but  as 
soon  as  the  cloud  disappears,  as  at  the  altitude  F,  the  further  descent  is 
accompanied  by  a  rapid  increase  of  temperature  almost  at  the  normal  adiabatic 
rate,  FG.  On  reaching  the  lower  valleys,  a  temperature  H  is  attained,  which 
is  greatly  in  excess  of  that  of  the  air,  A,  in  the  northern  valleys  before  the 
foehn  began  to  blow,  and  a  number  of  degrees  higher  than  the  temperature  of 
the  Italian  air  when  its  ascent  began  on  the  further  side  of  the  mountains.  At 
the  same  time,  the  air  will  have  become  extremely  dry;  from  being  saturated 
at  the  height  F,  its  dew-point  comes  to  be  HK  degrees  below  its  temperature 
in  the  valley  bottom. 

The  increase  of  temperature  produced  by  this  peculiar  reaction  of  latent 
heat  will  be  stronger  if  the  air  is  damp  and  relatively  warm  before  beginning 
the  ascent  of  the  mountains,  and  under  such  conditions  this  second  cause  of 
the  heat  and  dryness  of  the  foehn  may  be  as  effective  as  the  first ;  but  it  must 
be  remembered  that  in  the  case  of  several  foehn  winds  studied  in  Switzerland, 
the  simple  descent  of  the  upper  winter  air  is  the  first  cause  of  the  heat  and 
dryness  of  the  wind,  and  the  liberation  of  latent  heat  is  only  a  later  and 
secondary  cause ;  there  sometimes  being  no  rainfall  on  the  southern  slopes 
until  a  day  or  more  after  the  fully-developed  foehn  is  felt  in  the  northern 
valleys. 

The  heat  and  dryness  of  the  foehn  are  so  unlike  the  cold  and  dampness  of 
the  wind  on  the  mountain  passes  that  it  was  only  natural  for  earlier  observers 
to  ascribe  the  origin  of  the  warm  wind  to  some  warm  source ;  and  it  was  con- 
sequently referred  to  the  hot  Sahara  and  regarded  as  an  extension  of  the  sirocco 
of  southern  Italy.  This  has  been  completely  disproved.  Espy  and  Dove  were 
among  the  earlier  meteorologists  who  suggested  that  the  changes  of  temperature 
in  vertical  currents  and  the  liberation  of  the  latent  heat  from  condensing  vapor 
must  be  considered  in  its  explanation  ;  and  this  has  been  fully  confifmed  by 
modern  students  ;  especially  by  Hann  of  Vienna,  to  whom  the  suggestion  of 
the  initial  cause  of  the  heat  and  dryness  of  the  foehn  is  due,  and  whose  studies 


242  ELEMENTARY    METEOROLOGY. 

have  given  careful  demonstration  of  the  more  general  suggestions  of  others. 
When  thus  explained,  it  is  manifest  that  the  foehn  need  not  be  limited  to  a 
south  wind  in  the  northern  valleys  of  the  Alps ;  it  might  occur  under  fitting 
conditions  as  a  north  wind  in  the  southern  valleys  of  those  mountains,  and 
such  has  proved  to  be  the  case ;  it  might  occur  at  the  base  of  other  mountain 
ranges,  as  has  been  abundantly  shown.  Wherever  a  lively  cyclonic  indraft 
draws  the  air  away  from  the  valleys  at  the  foot  of  lofty  mountains,  leaving 
them  to  be  filled  by  the  descent  of  air  from  the  altitude  of  the  mountain  crests 
and  later  by  the  passage  of  air  over  the  range,  a  foehn-like  wind  may  be 
expected. 

These  conditions  are  admirably  developed  along  the  eastern  base  of  our 
Kocky  Mountains  in  Montana,  Wyoming,  and  Colorado,  as  well  as  in  the 
Northwest  Territories  of  Canada.  It  frequently  happens  in  the  winter  season 
that  as  a  cyclonic  center  moves  eastward  from  British  Columbia  to  Manitoba, 
while  an  anticyclone  follows  across  Utah,  an  extended  cyclonic  circulation  is 
developed  over  the  mountain  region.  The  winds  that  blow  northward  along 
the  plains,  as  the  cyclone  advances,  are  soon  supplanted  by  westerly  winds  ; 
and  as  these  descend  from  the  mountains  and  flow  out  upon  the  plains,  all  the 
features  of  the  Swiss  foehn  are  developed.  The  warm  and  dry  wind  thus 
produced  over  a  belt  of  country  along  the  foot  of  the  mountains  is  called  the 
chinook.  While  the  west  wind  may  be  damp  and  chilly  under  heavy  clouds 
with  a  plentiful  fall  of  rain  or  snow  on  the  western  side  of  the  Front  Range, 
the  sky  is  fair  or  clear  over  the  plains,  and  the  clouds  are  left  behind  at  the 
summit  of  the  mountains ;  and  although  the  velocity  of  the  wind  east  of  the 
mountains  may  be  high,  and  its  arrival  may  be  at  night,  its  temperature  is 
mild  or  even  warm,  in  marked  contrast  with  the  colder  air  that  it  has  displaced 
and  with  the  cold  wave  that  usually  follows  a  few  days  later.  The  warm 
chinook  may  arrive  at  night  as  well  as  by  day  (see  Fig.  10,  curve  y) ;  it  quickly 
melts  or  dries  up  the  snows  of  preceding  storms,  thus  laying  bare  the  northern 
plains  and  enabling  cattle  to  survive  the  winter  without  protection.  An 
isolated  area  of  relatively  high  temperature  may  thus  be  produced  by  an 
extended  chinook  along  the  eastern  base  of  the  Rocky  Mountains.  Owen's 
Valley,  below  the  Sierra  Nevada  in  eastern  California,  should  possess  well- 
developed  chinook  winds.  It  lies  to  leeward  of  a  lofty  mountain  range  whose 
western  slope  is  visited  by  heavy  snow  storms  in  the  winter  season  ;  but  as 
yet  no  accounts  of  such  winds  have  been  received  from  that  nearly  desert 
region. 

<  >ne  of  the  most  remarkable  examples  of  the  foehn  is  found  on  the  western 
coast  of  Greenland,  where  the  cyclonic  wind  descending  from  the  icy  plateau 
of  the  interior  becomes  unduly  warm  and  dry,  raising  the  temperature  at 
sea  level  even  thirty  or  forty  degrees  above  the  winter  mean.  One  of  the  best 
examples  of  its  occurrence  was  during  nine  consecutive  days  in  November  and 


CYCLONIC    STORMS   AND    WINDS. 


243 


December,  1875,  when  it  was  as  warm  in  western  Greenland  as  in  northern 
Italy,  and  warmer  than  in  Canada,  Iceland,  Great  Britain,  and  over  the 
intervening  Atlantic  ;  at  one  station  of  observation  it  was  warmer  in  the 
darkness  of  the  polar  night  than  in  France  at  noon-day. 

The  southern  hemisphere  has  well-marked  foehns  in  New  Zealand,  where 
they  descend  from  the  mountains  and  blow  as  "northwesters"  across  the 
Canterbury  plains ;  and  again  at  the  eastern  base  of  the  Andes  in  the 
Argentine  Kepublic,  where  their  occurrence  presumably  on  the  equatorward 
side  of  passing  cyclones  is  symmetrical  with  that  of  the  chinook  in  our 
western  states. 


7000 


249.  The  anticyclonic  calm.  Although  it  is  not  at  first  apparent  why 
calm  air  should  be  associated  with  a  group  of  winds,  still  under  a  natural 
classification  it  seems  best  to  place  anticyclones,  in  which  the  winds  are  very 
gentle  or  at  rest,  in  close  association  with  the  more  boisterous  cyclonic  move- 
ments of  the  atmosphere  just  considered.  Anticyclones  possess  a  gentle 
descending  movement  of  the  air,  and  a  quiet  marginal  outflow  near  the  earth's 
surface,  as  has  been  explained  in  Section  238.  They  are  in  consequence  pre- 
vailingly clear  or  fair  and  dry  atmospheric  regions ;  and  in  this  respect  they 
are  strongly  unlike  the  cloudy  and  wet  air  of  cyclonic  areas.  'But  as  their 
movements  are  of  a  definite  kind,  so  are  their  various  other  features ;  and 
hence  they  deserve  consideration  as  phenomena  having  persistent  and  recog- 
nizable characteristics.  Being  prevail- 
ingly clear,  their  surface  air  takes  on 
the  features  of  their  season,  and  they 
must  therefore  be  considered  under 
different  heads  for  winter  and  summer. 

In  summer  time,  when  the  mean 
vertical  temperature  gradient  is  AB, 
Fig.  86,  the  vertical  temperature  gradi- 
ent of  the  greater  mass  of  the  descend- 
ing anticyclonic  air  nearly  coincides 
with  the  adiabatic  line  CD,  but  the 
air  near  the  ground  has  strong  diurnal 
range  of  temperature,  as  indicated  by 
the  downward  divergence  of  the  dotted 
lines  DR  and  DN-,  rising  to  a  high 
temperature  by  day,  and  cooling 
rapidly  at  night.  The  low  nocturnal  + 
temperature  generally  produces  fog 

in  the  lowlands ;  but  the  heat  of  the  surface  air  in  the  day-time  soon  dissipates 
the  fog  and  causes  active  local  convection,  generally  producing  fair-weather 


244 


ELEMENTARY    METEOROLOGY. 


cumulus  clouds  ;  the  heat  is  sometimes  excessive  enough  to  give  rise  to  local 
t, Hinder  storms,  which  will  be  considered  in  the  next  chapter.  At  night,  the 
clouds  all  dissolve  away,  the  air  is  habitually  calm,  and  owing  to  the  rapid 
cooling  of  the  lower  air  by  nocturnal  radiation,  there  is  a  distinct  inversion  of 
temperature  perceptible  between  valleys  and  hills  ;  and  the  minimum  on  anti- 
cyclonic  nights  generally  furnishes  the  lowest  temperature  of  the  month. 

In  winter  time,  anticyclones  present  very  different  conditions.  The  days 
then  are  short  and  the  sun  rises  little  above  the  horizon  ;  insolation  is  brief 
and  weak,  and  the  temperature  of  the  lower  air  in  the  clear  space  of  anti- 
cyclones is  dominated  chiefly  by  radiation.  The  ground  at  such  times, 
especially  if  snow  covered,  becomes  excessively  cold  and  the  air  near  it  is 
greatly  cooled  by  radiation  and  conduction.  The  baric  gradients  being  faint, 
the  air  lies  nearly  quiet  and  becomes  colder  and  colder  as  long  as  the  anti- 
cyclone endures.  Thus  the  low  temperatures  imported  by  a  cold  wave  are 
often  followed  by  still  lower  minima  locally  produced  in  an  anticyclonic  calm. 
The  cooling  may  become  so  excessive  that,  in  spite  of  the  small  absolute 
humidity  of  the  air,  its  dew-point  is  reached  and  then  its  lower  layers  are 
charged  with  a  cold  chilling  ground  fog.  It  is  however  only  on  land  and  in 

the  lower  air  that  these  extreme 
features  are  developed.  In  the  middle 
layers  of  the  atmosphere,  the  air  is 
neither  cold  nor  foggy.  Recalling  the 
slow  descent  of  the  anticyclonic  air  and 
assuming  AB,  Fig.  87,  as  the  mean 
vertical  temperature  gradient  of  our 
winter  season,  it  follows  that  the  verti- 
cal temperature  gradient  in  winter- 
anticyclones,  above  the  reach  of  the 
stronger  influence  of  the  cold  surface 
of  the  earth,  will,  as  in  summer  anti- 
cyclones, approach  close  to  the  adi- 
abatic  line,  CD,  and  thus  produce  an 
extraordinary  contrast  between  the 
temperature  and  humidity  of  the 
middle  and  lower  air,  illustrated  by 
the  excessive  inversions  of  temperature 
of  such  occasions.  The  air  descending  from  altitudes  above  the  limit  of  the 
figure  has  at  heights  above  2,500  feet  a  temperature  gradient  such  as  EH, 
which  departs  but  little  from  the  adiabatic  gradient,  CD;  this  indicates  a 
temperature  decidedly  above  the  normal  of  the  season.  Mountain  observations 
show  that  the  increase  of  temperature  may  take  place  during  the  night,  and 
without  perceptible  movement  of  the  air.  Nearer  the  ground,  the  tendency  to 


:•:. 


\ 


CYCLONIC    STORMS    AND    WINDS.  245 

increase  of  temperature  by  compression  during  the  slow  and  almost  stagnating 
descent  of  the  air,  is  overcome  by  radiation  and  conduction  to  the  extremely 
cold  surface  of  the  ground,  and  the  gradient  becomes  HG.  The  upper  air 
being  extremely  dry,  we  may  suppose  that  at  a  height  of  7,000  feet,  where 
the  temperature  is  E,  the  dew-point  is  e.  The  rise  of  the  dew-point  in  the 
descending  air  on  account  of  compression  alone  would  cause  it  to  change 
during  descent  at  the  slow  rate,  ed ;  but  if  a  slight  addition  of  humidity  is 
caused  by  the  diffusion  of  vapor  upward  into  the  slowly  settling  air,  the  dew- 
point  will  rise  at  the  faster  rate,  eF.  The  intersection  of  EHG  and  eF  will 
indicate  the  altitude  and  temperature  of  the  dew-point  in  the  lower  part  of  the 
anticyclonic  mass ;  and  below  this  the  air  will  be  foggy.  The  radiation 
which  before  took  place  from  the  ground  is  then  continued,  perhaps  less 
effectively,  from  the  upper  part  of  the  fog  stratum,  and  its  thickness  increases. 
Thus  while  an  observer  on  a  mountain  peak  finds  the  air  in  winter  anti- 
cyclones of  relatively  mild  temperature  and  pleasant  dryness  under  a  clear 
sky,  he  may  look  down  on  a  sea  of  fog  submerging  the  lowlands,  where  all  is 
cold,  chilling,  damp,  and  dark.  Unlike  the  fair-weather  valley  fogs  of  summer 
nights,  these  winter  fogs  survive  the,  day-time  also,  and  may  endure  for  a  week 
in  regions  where  the  progression  of  the  anticyclones  is  slow,  as  in  central 
Europe. 

It  should  be  remembered  that  the  strong  contrasts  of  temperature  here 
considered  are  not  alone  the  result  of  cooling  in  the  lower  air,  but  of  warming 
in  the  middle  layers  as  well ;  and  further,  that  the  mountains,  on  which  obser- 
vations at  higher  levels  are  generally  made,  have  nothing  to  do  with  producing 
the  contrast,  but  serve  only  as  convenient  points  for  observation.  Indeed,  the 
mountain  mass  must  serve  to  diminish  somewhat  the  temperature  of  the  air 
that  settles  upon  it ;  observations  from  balloons  would  probably  give  stronger 
contrasts  than  those  known  from  mountain  stations ;  but  balloon  ascents  are 
seldom  made  in  the  winter  season. 

Several  examples  of  anticyclonic  inversions  in  winter  may  be  introduced. 
On  December  27,  1884,  when  the  pressure  was  high  and  the  wind  over  New 
England  was  everywhere  light,  the  early  morning  temperature  on  Mount 
Washington  was  +16°,  while  the  records  in  the  surrounding  lowlands  ranged 
from —'10°  to —24°;  thus  showing  that  the  reversed  curvature  of  the  line 
/;//<;,  Fig.  87,  is  probably  not  exaggerated.  Even  Pike's  Peak  (14,134')  shows 
occasional  higher  winter  temperatures  then  Denver  (5,291')  (see  Fig.  12,  c,  d), 
and  these  appear  in  all  cases  to  be  associated  with  anticyclones. 

The  most  remarkable  example  of  anticyclonic  inversions  of  temperature 
yet  studied  occurred  in  Europe  in  December,  1879.  The  pressure  of  the 
month,  reduced  to  sea  level,  averaged  over  770  mm.  (30.32  in.)  over  Austria, 
Germany,  Switzerland  and  France,  with  lower  pressures  on  all  the  surrounding 
regions  ;  thus  indicating  persistent  anticyclonic  conditions  for  this  period  ;  and 


246  ELEMENTARY   METEOROLOGY. 

the  temperature  in  central  Austria  and  Bavaria  for  the  same  time  averaged 
less  than  —  10°  centigrade  (-{- 14°  F.).  The  lowlands  were  shrouded  in  fog  for 
much  of  the  month,  while  the  mountain  stations  reported  persistently  clear 
weather  and  mild  temperatures,  over  twenty  Fahrenheit  degrees  warmer  than 
the  valleys  5,000  feet  below  them.  The  view  from  certain  mountain  summits 
was  described  as  extending  over  a  broad,  smooth  sea  of  cloud  which  concealed 
all  the  low  country,  while  all  the  lofty  mountains  and  here  and  there  the 
higher  hills  rose  above  it  like  islands. 

It  is  evident  that  the  temperature  on  low  ground  during  an  anticyclone 
differs  from  that  in  the  presence  of  a  foehn  on  account  of  the  difference  in  the 
velocity  of  descent  of  the  air  in  the  two  cases.  The  foehn  rushes  down  so 
rapidly  that  it  loses  by  radiation  and  conduction  but  little  of  the  heat  that  it 
gains  by  compression  ;  the  anticyclonic  air  settles  down  so  slowly  that  it 
cannot  preserve  the  heat  that  comes  from  its  compression,  and  therefore  suffers 
its  temperature  to  be  controlled  by  processes  of  cooling  as  it  approaches  the 
earth.  It  is  also  evident  that  all  velocities  of  descent  should  be  represented 
in  the  great  variety  of  weather  conditions  between  the  extremes  of  these  two 
classes  of  phenomena.  If  a  foehn  occurs  with  but  moderate  velocity,  the 
peculiar  features  of  its  class  will  be  faintly  developed.  If  a  local  breeze 
occurs  for  some  reason  in  an  anticyclonic  area,  presumably  accompanied  by  a 
more  rapid  descent  than  usual  of  the  overlying  air,  cooling  by  radiation  will 
have  less  time  to  counteract  warming  by  compression,  and  a  rise  of  tempera- 
ture would  be  noted.  Precisely  this  case  has  been  detected  in  Austria,  and  it 
doubtless  will  yet  be  discovered  in  this  country  and  in  the  interior  of  Canada. 
Like  the  bora  that  may  be  expected  on  the  slopes  of  the  plateaus  of  Utah  and 
Arizona,  the  warmer  breezes  within  the  extremely  cold  areas  of  anticyclones 
should  be  critically  looked  for  by  observers  favorably  situated  for  their 
recognition. 

250.  Comparison  of  the  foregoing  examples.  The  well  marked  features 
of  the  several  classes  of  cyclonic  winds  and  of  the  anticyclonic  calm  may  now 
be  briefly  reviewed.  The  sirocco  is  warm  because  it  comes  from  a  warm 
region ;  it  is  dry  if  derived  from  arid  regions,  or  moist  if  flowing  from  warm 
seas  into  cyclonic  centers.  The  cold  wave  is  cold  because  it  comes  from  a  cold 
region ;  it  is  relatively  dry  because  its  temperature  rises  as  it  advances.  The 
bora  is  cold  in  spite  of  its  descent,  because  it  was  unduly  cold  before  the 
descent  began.  The  foehn  is  warm  and  dry  on  account  of  its  supply  from 
high  levels  at  moderate  temperatures  and  its  rapid  descent  to  lower  levels. 
Tin-  inversion  of  temperature  in  winter  anticyclones  is  due  to  the  slow  descent 
of  their  central  air ;  thus  allowing  the  production  of  relatively  high  tempera- 
ture and  low  humidity  at  middle  elevations,  and  of  extremely  low  temperature 
and  high  humidity  at  low  levels.  Although  these  various  classes  may  be  con- 


CYCLONIC    STORMS    AND    WINDS.  247 

nected  with  one  another  by  intermediate  examples,  they  are  all  easily 
recognized  when  well  developed,  and  hence  serve  as  convenient  types  with 
which  our  many  kinds  of  weather  may  be  compared.  They  are  not  normal 
members  of  the  general  circulation,  but  are  products  of  disturbances  that 
interrupt  the  steadier  flow  of  the  great  body  of  the  atmosphere  ;  they  do  not 
involve  the  higher  levels  of  the  air,  but  are  for  the  most  part  developed  in 
greatest  distinctness  close  to  the  surface  of  the  earth  on  which  we  live. 


248 


ELEMENTARY    METEOROLOGY. 


CHAPTER   XL 

LOCAL,  STORMS. 
THUNDER  STORMS. 

251.  Thunder  storms  and  thunder  squalls.    Lightning  is  seen  and  thundei 
is  heard  in  many  rain  storms  that  do  not  present  the  peculiarities  of  those  to 
which  the  name,  thunder  storm,  is  best  applied.      Tropical  hurricanes  for 
example  are  accompanied  by  violent  electrical  manifestations  near  their  centers, 
but  they  are  not  for  this  reason  to  be  called  thunder  storms.     Light  falls  of 
rain  in  the  spring  and  summer  are  not  infrequently  in  our  country  accompanied 
by  moderate  lightning  and  thunder ;  but  these  hardly  deserve  a  stronger  name 
than  thunder  showers.     The  typical  and  fully-developed  thunder  storm,  with 
its  peculiar  outrushing  squall  or  gust  of  wind,  is  a  violent  and  relatively  local 
disturbance,    which  certain  well-marked  features  of  cloud  form  distinguish 
clearly  enough  from  other  kinds  of  storms.    Such  storms  occur  chiefly  in  warm 
regions,  in  the  warm  season  and  in  the  afternoon  or  early  evening. 

252.  The  passage  of  a  thunder  storm.     The  coming  of  a  well-formed 
summer  thunder  storm   is   heralded  by  a  forerunning  layer  of  cirro-stratus 
cloud,  c,  c,  Fig.  88,  commonly  appearing  in  the  west  during  the  afternoon ; 
fibrous  or  hazy  at  the  forward  edge,  growing  thicker  and  sometimes  showing 
smaller  or  larger  festoons  (/,  /),  slowly  descending  and  dissolving  from  its 


FIG.  88a. 

under  surface  as  the  great  rain-bearing  cloud  mass  (n)  approaches.  A  group 
of  such  festoons,  observed  near  Philadelphia,  July  16, 1887,  is  given  in  Fig.  Si). 
The  cirro-stratus  cover  may  extend  from  ten  to  fifty  miles  in  advance  of  the 
rain.  The  temperature  preceding  the  storm  is  as  a  rule  oppressively  high, 
with  light  southerly  winds,  which  have  prevailed  during  an  antecedent  dry 
spell  ;  but  there  is  a  slight  cooling  as  the  forerunning  cloud  cover  spreads  over 


LOCAL    SSTOUMS. 


1>49 


the  sky  and  hides  the  sun.  Perhaps  an  hour  after  the  first  sight  of  the  cirro- 
stratus  sheet,  one  may  see  heavy  cumulo-nimbus  clouds  or  "  thunder  heads  " 
(t)  of  a  dull  leaden  color  and  threatening  appearance  rise  underneath  it  on  the 


FIG.  89. 

western  horizon.  Distant  thunder  is  heard  as  the  thunder  heads  come  nearer ; 
and  then  their  low  level  base  (b)  may  be  seen,  below  which  the  gray  rain 
curtain  (r)  trails  to  the  ground  and  conceals  all  objects  behind  it ;  the  height 
of  the  clouds  at  their  base  being  but  an  eighth  or  a  tenth  of  their  summit 
height.  Smaller  detached  clouds  (d)  often  form  in  front  of  the  main  mass, 
drifting  into  it  and  increasing  in  size  as  they  coalesce  with  the  storm  cloud- 
The  entrance  of  these  detached  clouds  (d)  and  the  flow  of  the  lower  winds  (b) 


FIG.  886. 


into  the  great  storm  cloud  does  not  necessarily  imply  a  westward  motion. 
They  may  move  to  the  east ;  for  the  storm  is  advancing  eastward  and  will 
overtake  them  if  their  movement  is  slower  than  its  own.  A  ragged  light  gray 
"  squall  cloud  "  (s)  rolls  beneath  the  great  dark  cloud  mass,  a  little  behind  its 
forward  edge ;  and  the  whole  structure  advances  broadside  across  country  at 


250  ELEMENTARY    METEOROLOGY. 

a  rate  of  from  twenty  to  fifty  miles  an  hour.  Below  the  clouds  and  in  front 
of  the  rain  is  the  short-lived  outrushing  wind  squall  (y),  brushing  up  the  dust 
that  has  been  parched  in  the  preceding  drought ;  a  cool  blast  in  strong  contrast 
with  the  relatively  stagnant  hot  southerly  air  that  preceded  the  storm.  The 
temperature  may  fall  ten  or  twenty  degrees  in  twice  as  many  minutes  when 
the  squall  arrives,  as  appears  in  the  tracing  of  a  thermograph  record,  Fig.  lit/, 
at  Providence,  R.  I.,  for  July  21,  1885.  The  barometer,  that  has  been  falling 
slowly  up  to  this  time,  suddenly  rises  about  five  hundredths  of  an  inch  as  the 
squall  arrives  ;  then  the  wind  soon  weakens,  and  the  barometer  after  standing 
steady  or  falling  slightly,  gradually  rises  as  the  storm  breaks  away.  The  rain 
begins  in  large  pelting  drops  shortly  after  the  onset  of  the  squall,  and  soon 
increases  to  a  heavy  downpour,  often  with  hail ;  and  at  the  same  time  the 
humidity  rapidly  increases  almost  or  quite  to  Saturation.  The  thunder, 
growing  louder  while  the  rain  approached,  now  follows  quickly  after  vivid 
flashes  of  lightning ;  and  by  this  time  the  darkness  of  the  shadow  in  front  of 
the  rain  is  already  diminishing.  The  storm  moves  rapidly  across  country,  and 
in  half  an  hour,  more  or  less,  the  rain  slackens  and  the  clouds  break  in  the 
west.  As  they  drift  away,  the  pure  blue  sky  is  seen  in  the  rear  of  the  storm, 
and  a  little  later  the  rainbow  springs  over  the  eastern  horizon  on  the  after  side 
of  the  rain  curtain,  opposite  the  low  sun  near  its  setting  in  the  west.  The  air 
is  left  cooler  and  cleaner  than  before  ;  and  the  refreshed  colors  of  the  landscape, 
brightening  towards  sunset,  form  a  most  grateful  contrast  to  the  hot  glare  of 
noon  and  to  the  dark  uproar  of  the  storm.  As  the  storm  recedes  and  its 
thunder  dies  away,  the  rear  of  its  festooned  or  bracketed  overflow  (k)  may  be 
seen  far  in  the  east,  reaching  somewhat  backward  from  the  top  of  the  great 
cloud  mass,  delicately  tinted  by  the  rays  of  the  setting  sun.  The  clouds  reach 
a  mountainous  height,  and  retain  a  pink  glow  after  their  base  is  lost  in  the 
dull  blue  sky  underneath  the  twilight  arch. 

253.  Observation  of  thunder  storms.  Observations  at  single  stations  serve 
to  give  the  hours  and  seasons  when  thunder  storms  are  most  frequent  j  and  it  is 
thus  found  that  a  decided  excess  of  storms  occurs  during  the  warmer  spells  of 
the  warm  season  and  in  the  afternoon  or  early  evening  hours  ;  but  this  kind 
of  observation  does  not  suffice  for  the  determination  of  their  larger  features. 
For  this  reason,  the  systematic  study  of  thunder  storms  by  numerous  volunteer 
observers,  all  working  on  a  uniform  plan,  has  been  attempted  in  the  various 
countries  of  Europe  and  in  different  parts  of  the  United  States,  with  the  most 
interesting  results.  The  arrival  of  the  squall  wind,  the  beginning  and  ending 
of  the  rain,  and  the  time  of  heaviest  thunder,  serve  to  mark  the  arrival  and 
passage  of  the  storm  with  much  accuracy.  The  accounts  of  local  storms,  when 
reported  for  newspapers,  should  give  at  least  some  of  these  data,  as  well  as  a 
statement  of  the  damage  done  by  wind,  rain,  or  lightning.  When  the  records 


LOCAL    STORMS. 


251 


FIG.  90. 


from  many  stations  are  charted,  and  lines  are  drawn  to  indicate  the  place  of 
the  storm  front  at  successive  half  hours  or  hours,  a  number  of  isolated  storms 
of  moderate  size  may  be  detected,  all  moving  in  a  common  direction ;  or  a 
single  large  thunder  storm  may  be  found,  advancing  broadside  or  obliquely, 
with  a  belt  of  clouds  fifty  or  a  hundred  or  more  miles  in  length  and  from  ten 
to  thirty  miles  in  breadth,  exclusive  of  the  cirro-stratus  cover  which  in  large 
storms  spreads  out  many  miles  in  advance  of  the  rain-cloud  belt.  The  height 
of  the  cloud  tops  certainly  reaches  five  miles  in  our  stronger  summer  thunder 
storms,  the  upper  clouds  being  then  of  ice 
crystals  in  spite  of  the  high  temperature  at 
the  bottom  of  the  atmosphere.  The  follow- 
ing diagrams  taken  from  various  sources 
illustrate  well-marked  examples  of  thunder 
storms  in  this  country  and  abroad. 

Fig.  90  is  reproduced  from  the  earliest 
map  of  a  storm  of  this  kind  in  the  United 
States,  made  by  Hinrichs  no  longer  ago 
than  July,  1877.  It  occurred  in  Iowa,  where 
numerous  local  observations  were  used  to  define  its  advance.  The  storm  front 
became  wider  as  it  moved  forward  ;  the  heaviest  rainfall  lay  along  the  axis  of 
the  storm,  amounting  to  two  inches  in  the  southeastern  part  of  the  state  ;  and 
the  outrushing  wind  squall  was  of  destructive  strength  at  many  places.  A 

thunder  squall,  charted  by  Clayton, 
Fig.  91,  began  in  northeastern  Missouri 
about  noon  on  July  5,  1884,  and  ran 
southeastward  with  convex  front  at 
a  rate  of  a  little  over  fifty  miles  an 
hour,  until  it  faded  away  in  northern 
Georgia  about  midnight.  A  small 
thunder  squall  of  much  intensity  was 
observed  to  cross  New  England  on 
July  21,  1885,  Fig.  92,  leaving  the 
Hudson  valley  about  ten  o'clock  in 
the  morning,  and  reaching  the  ocean 
about  an  hour  after  noon.  Its  clouds 
were  seen  and  its  thunder  was  heard 
by  observers  to  the  north  and  south  of  its  path  where  no  rain  fell.  Beneath 
its  dark  shadow  there  was  a  rapid  fall  of  temperature  while  its  brief  rain 
shower  lasted,  as  illustrated  in  Fig.  Ha ;  but  southerly  winds  and  high 
temperatures  were  resumed  after  its  passage,  and  maintained  until  the  arrival 
of  a  much  larger  storm  in  the  late  afternoon. 


Va. 


FIG.  91. 


252 


ELEMENTARY    METEOROLOGY. 


FIG.  !»•_>. 

A  remarkable  thunder  storm  traversed  northern  and  central  Germany  on 
August  9, 1881.  the  hourly  positions  of  its  somewhat  discontinuous  front  being 

charted  by  Koppen  in  Fig. 
93,  from  nine  o'clock  in  the 
morning  till  nine  in  the 
evening.  It  gave  few  light- 
ning strokes,  but  brought 
heavy  rain,  with  hail  at 
some  places,  and  a  violent 
outrushing  squall,  lasting 
five  or  ten  minutes  and 
doing  much  damage  during 
its  brief  outburst.  Light 
southerly  winds  with  higli 
temperature  (85°)  occupied 
the  region  in  advance  of 
the  storm;  cooler  westerly 
winds  (70°)  followed  it; 
the  contrast  of  temperature 
being  abrupt  on  either  side 
Flo  93  of  the  rain  front.  As  a 

rule,  however,  the  tlniiu lei- 
storms  of  Europe  are  less  extended  and  less  violent  than  those  of  the 
Mississippi  valley. 

254.  Convectional  action  in  thunder  storms.  All  the  features  of  thunder 
storms  point  to  their  dependence  on  a  convectional  overturning  of  the 
atmosphere.  They  occur  in  warm  regions  and  in  the  warm  season,  when 


LOCAL    STORMS. 


253 


FIG.  94. 


Fi«.  95. 


the  vertical  temperature  gradient  is  stronger  than  in  cold  regions  or  in  winter. 
They  are  most  common  and  most  violent  in  spells  of  warm  summer  weather, 
and  at  or  shortly  after  the  hour  of  the  day  when  con- 
vectional  movements  are  most  active.  They  receive 
great  assistance  from  the  latent  heat  liberated  from 
their  abundant  condensation  of  vapor  at  relatively  high 
temperatures.  Their  early  stages  may  be  traced  back 
to  a  beginning  in  ordinary  cumulus  clouds,  in  which  the 
misty  filaments  are  seen  ascending  and  inflowing  at  the 
base,  but  rolling  out  and  dissolving 
at  the  top  on  the  side  where  the 
general  motion  of  the  air  currents 
brushes  them  forward.  Many  such 
clouds  may  be  watched  during  a  sum- 
mer morning  from  their  first  appear- 
ance, through  their  later  growth  to  a 
large  size,  and  then  to  their  fading 
away;  all  these  changes  often 
requiring  but  a  fraction  of  an 
hour,  in  which  the  cloud  re- 
mains clearly  in  sight  as  it 
floats  from  west  to  east.  The 
warmer  the  day,  the  larger  the 
clouds  and  the  longer  their  life. 
About  noon,  or  soon  after  it,  the  attentive  observer  may  sometimes  notice  that 
some  of  these  towering  cumulus  clouds  reach  a  greater  size  than  the  rest,  their 
ragged  lower  edges  still  showing  inflowing  wisps,  while  the  sharp-cut  convex 
summits  mount  higher  and  higher,  and  at  last  manifest  the  initial  stages  of 
cirro-stratus  overflow.  The  cloud  then  assumes  the  familiar  anvil  form,  so 
commonly  associated  with  distant  thunder  storms.  Figs.  94  to  97  illustrate  a 
series  of  such  changes,  by  which  an  ordinary  cumulus  cloud  is  transformed 
into  a  thunder  storm  nimbus  :  they  were  sketched  when  looking  northward 
from  near  New  York  city  at  11.00,  11.15,  11.40,  and  12.45  o'clock  on  July  2, 
1887.  The  storm  drifted 
to  the  northeast,  and 
passed  out  of  sight. 

Even  after  the  first 
cirrus  overflow  takes 
place,  the  cloud  may  fail 

to  produce  much  rain,  unless  the  process  of  growth  is  actively  continued  ;  but 
if  the  air  be  especially  hot  and  sultry,  a  full  development  is  likely  to  follow 
such  a  beginning.  If  an  overflowing  cloud  of  this  kind  comes  in  sight  over 


254  ELEMENTARY  METEOROLOGY. 

the  western  horizon  shortly  after  noon,  floating  eastward  in  the  upper  winds, 
its  further  increase  to  mature  size  and  strength  may  generally  be  seen  as  it 
passes  the  observer.  The  inflow  at  the  forward  lower  edge  is  easily  recognized 
by  the  movement  of  the  cloud  wisps  ;  the  ascent  of  the  currents  within  the 
great  cloud  mass  is  clearly  demonstrated  by  the  upward  expansion  of  the  lofty 
thunder  heads  ;  the  outflow  at  the  summit  is  manifested  in  the  cirro-stratus 
sheet,  presumably  beginning  at  an  altitude  where  the  ascending  air  is  cooled 
to  the  temperature  of 'the  air  around  it.  Sometimes  one  part  of  the  cloud 
may  reach  a  somewhat  greater  height  than  the  rest,  as  if  supplied  by  a  rather 
warmer  or  moister  indraft  at  the  base.  When  the  summit  outflow  is  well 
established,  it  flows  forward  in  the  faster-moving  upper  currents  ;  sometimes 
toppling  over  tumultuously,  and  dissolving  away  as  it  settles  to  lower  levels  ; 
sometimes  spreading  evenly  eastward  in  a  broad  sheet,  more  or  less  distinctly 
festooned  on  its  under  surface. 

The  first  trails  of  rain  fall  from  the  base  of  the  cloud  and  the  first  peals  of 
thunder  are  heard  from  within  it  at  about  the  time  that  the  top  of  the  cloud 
begins  to  spread  out ;  presumably  because  the  phange  from  a  nearly  vertical 
ascent  to  a  horizontal  outflow  fails  to  support  the  cloud  particles  ;  they  fall 
through  the  cloud,  increasing  in  size,  and  reach  the  ground  as  large  drops  of 
rain.  The  occurrence  of  hail  is  also  indicative  of  active  convectional  motion, 
as  will  be  more  fully  explained  in  Section  279.  Sometimes  many  separate 
thunder  clouds  move  eastward  at  a  common  speed  ;  at  other  times,  many  clouds 
isolated  at  first  seem  to  coalesce  and  form  the  long  cloud  belts  of  the  large 
thunder  storms  described  above.  The  constriction  of  the  cloud  mass  between 
its  broad  base  and  its  still  broader  overflow,  producing  the  anvil  form,  has  given 
rise  to  the  idea  that  thunder  storms  consist  of  two  cloud  layers,  between  which 
the  lightning  flashes  and  the  hailstones  rise  and  fall ;  but  this  is  now  contra- 
dicted by  many  observations.  The  side  view  of  distant  storms  shows  their 
cloud  mass  to  be  continuous  from  base  to  summit  ;  and  observers  on  mountains 
or  in  balloons  make  no  report  of  a  clear  space  between  the  upper  and  lower 
levels  of  thunder  storm  clouds.  The  whole  mass  of  the  ascending  current 
forms  a  single  gigantic  cloud.  The  same  interpretation  applies  to  cyclonic, 
storms.  Their  cirrus  overflow  is  often  seen  at  a  considerable  height  above 
their  lower  cumuliform  clouds,  with  a  space  of  clear  air  between;  but  this 
characterizes  the  marginal  parts  of  the  storm  area,  and  nearer  the  center  it  is 
in  the  highest  degree  probable  that  the  lower  clouds  are  confluent  upwards 
with  the  upper  clouds,  as  illustrated  in  Fig.  60. 

255.  Geographical  distribution  of  thunder  storms.  Thunder  storms  are 
<-i»mmon  in  the  doldrums  all  around  the  equatorial  regions.  Here  they  gener- 
ally occur  in  the  afternoon  or  early  evening,  yielding  heavy  rain,  by  which 
the  ocean  surface  is  appreciably  freshened  for  a  time  and  its  average  salinity 


LOCAL    STORMS.  255 

is  decreased  as  compared  with  that  under  the  steady-going  trade  winds  (see 
Fig.  105).  Although  active  while  they  last,  the  thunder  storms  of  the  mid- 
ocean  doldrums  do  not  attain  the  terrific  violence  reached  by  thunder  storms 
on  or  near  the  equatorial  lands,  such  as  are  experienced  in  equatorial  Africa 
and  on  the  Atlantic  waters  to  the  west  of  the  continent.  As  far  as  these  are 
described,  they  seem  to  have,  in  a  more  intense  degree,  all  the  features  of  the 
stronger  thunder  squalls  of  our  summer  season.  They  are  most  violent  in  the 
late  afternoon  or  evening.  They  possess  massive  clouds,  drifting  along  in 
the  higher  winds ;  and  hence  advancing  westward  and  passing  from  the  land 
to  the  sea.  They  are  surmounted  by  a  cirro-stratus  outflow,  as  with  us.  On 
their  arrival,  the  wind  suddenly  shifts  and  blows  out  from  beneath  the  clouds 
with  great  fury  ;  and  for  this  reason  they  are  called  African  tornadoes.1  Their 
rain  is  heavy  and  is  accompanied  by  blinding  flashes  of  lightning  and  a 
deafening  roar  of  thunder.  They  last  only  a  short  time,  and  as  they  pass 
away,  the  atmosphere  returns  to  its  orderly  diurnal  changes,  by  which  the 
torrid  zone  is  so  strongly  characterized. 

African  tornadoes  advance  broadside  over  the  ocean  after  the  manner 
described  above  for  our  thunder  storms  ;  their  long  front,  stretching  across 
the  sky,  seems  to  be  higher  in  the  middle  than  at  the  ends  ;  hence  the  name, 
u  arched  squall,"  often  applied  to  storms  of  this  class.  In  India,  storms  of 
the  same  character  sweep  down  the  plains  of  the  Ganges  from  the  west  or 
northwest  in  the  hot  season,  and  are  there  called  "  nor'westers."  Further 
inland,  towards  the  desert  region  of  the  Indus,  the  amount  of  rain  falling  in 
such  storms  diminishes ;  but  the  clouds  are  formed,  and  the  squall  rushes  out 
from  beneath  them,  raising  a  dense  cloud  of  dust  from  the  parched  ground ; 
and  these  overturnings  are  consequently  called  "  dust  storms." 

It  is  probable  that  the  simoom  of  the  Sahara  and  of  Arabia,  although 
without  rain,  clouds  and  thunder,  is  of  similar  convectional  origin,  as  it  is 
characterized  by  a  rapidly-advancing  wind,  drifting  the  surface  sands  and 
raising  great  volumes  of  dust  over  the  deserts.  This,  however,  is  a  hot  wind, 
there  being  no  rain  to  cool  it,  and  its  temperature  being  greatly  increased  by 
the  heat  taken  from  the  drifting  sand  as  well  as  by  strong  sunshine.  It  has 
overwhelmed  caravans,  suffocating  both  men  and  beasts  ;  but  there  is  no 
reason  to  believe  in  its  supposed  poisonous  qualities.  The  excessive  heat, 
dryness  and  dustiness  of  the  air  are  its  dangerous  features.  It  should,  how- 
ever, be  added  that  certain  hot  winds  in  deserts,  described  as  simooms,  are 
not  explained  simply  by  referring  them  to  convectional  overturnings,  like 
thunder  storms  except  for  their  dryness. 

In  the  Argentine  Republic  and  Uruguay,  thunder  storms  of  great  energy 
are  observed  in  the  summer  season,  under  the  name  of  "  pamperos."  Their 
heavy  clouds  are  preceded  by  the  long  cirro-stratus  cover ;  their  nearer  advance 

1  See  note  on  this  name,  Section  266. 


256  ELEMENTARY   METEOROLOGY. 

discloses  the  broadside  approach  of  the  lower  horizontal  cloud  front,  apparently 
arching  from  horizon  to  horizon  ;  the  squall  brushes  up  a  frightful  cloud  of 
dust  from  the  dry  pampas,  and  this  is  shortly  followed  by  drenching  rain 
with  incessant  lightning  and  thunder.  These  storms  are  greatly  dreaded  by 
ship  masters  in  the  estuary  of  the  Rio  de  la  Plata. 

The  cloud-bursts l  of  our  arid  western  districts  are  only  exaggerated 
thunder  storms.  They  are  local  and  short-lived,  and  seem  to  result  from  the 
sudden  overturning  of  a  large  mass  of  unstable  atmosphere.  The  clouds  that 
accompany  these  storms  have  every  feature  indicative  of  a  convectional  origin, 
and,  as  with  us,  may  be  placed  at  the  end  of  a  well-continued  series,  beginning 
with  ordinary  cumulus  clouds ;  passing  then  to  moderate  thunder  showers, 
from  which  so  little  rain  falls  that  it  evaporates  on  its  way  down  through  the 
thirsty  lower  air,  and  hardly  a  drop  reaches  the  parched  ground ;  next  to  more 
active  local  thunder  storms  of  the  usual  type ;  and  all  these  culminating  in 
the  drenching  fall  of  waters  from  the  cloud-burst.  A  narrow  strip  of  country 
is  inundated  by  such  storms  for  a  short  distance ;  temporary  streams  then 
rush  down  channels  that  are  nearly  dry  at  other  times,  gathering  sand  and 
dust,  and  delivering  the  discharge  of  the  storm  to  the  main  valleys  in  dark, 
muddy  torrents,  many  miles  from  the  place  of  the  rainfall. 

256.  Mountain  thunder  storms.  The  ascending  valley  breezes  that  run 
up  the  slopes  of  mountains  by  day  frequently  become  energetic  enough  in  the 
summer  season  to  form  clouds  above  the  mountain  summit,  and  afternoon 
thunder  storms  are  often  generated  in  this  way.  They  drift  away  from  the 
mountain  over  which  their  formation  began,  and  the  rain  that  falls  from  them 
trails  down  to  the  lower  ground ;  but  they  seldom  survive  long,  unless 
other  conditions  favor  their  growth.  Storms  of  this  kind  are  well  known  in 
the  Alps  and  in  the  mountains  of  our  western  territory.  It  is  not  uncommon 
for  an  observer  in  the  desert  plains  between  the  mountain  ranges  of  Arizona 
to  see  several  active  thunder  showers  over  the  higher  peaks  ;  their  rain  may 
cause  a  rise  in  streams  from  the  mountains  and  enable  them  to  creep  further 
down  into  the  desert  before  being  lost  in  the  dry  sands ;  but  very  little  rain 
falls  on  the  plains  from  such  storms. 

An  interesting  example  of  the  combined  action  of  the  diurnal  sea  breeze 
and  the  valley  breeze  in  forming  a  thunder  cloud  was  recorded  by  a  sea-captain 
as  long  ago  as  1815  on  the  mountainous  island  of  Hawaii  in  the  North  Pacific 
ocean.  He  wrote  that  soon  after  the  sea  breeze  set  in,  about  nine  o'clock  in 
the  morning,  a  cloud  began  to  form  on  the  mountain  slopes,  surrounding  the 
lofty  volcanic  summit  in  the  center  of  the  island  in  the  form  of  a  ring,  like 
the  wooden  horizon  that  surrounds  the  artificial  globe  ;  and  rain  was  soon 

1  This  term  was  originally  restricted  to  rainfalls  of  even  greater  suddenness  and  volume 
than  those  to  which  it  is  now  commonly  applied  in  the  west. 


LOCAL    STORMS. 


257 


afterwards  seen  to  fall  in  torrents  from  the  cloud.  This  was  continued  over 
noon  ;  but  towards  evening,  when  the  sea  breeze  died  away,  the  rain  ceased 
and  the  cloud  soon  disappeared ;  the  air  then  remained  clear  until  the  next 
morning,  when  the  same  sequence  of  changes  would  begin  again  with  wonderful 
regularity.  The  mountain  stood  in  bold  relief,  and  from  where  the  ship  lay, 
a  little  off  shore,  the  summit  could  always  be  seen  above  the  cloud  even  when 
it  was  densest  and  blackest,  with  lightning  flashing  from  it,  as  happened  every 
day ;  the  rain  never  extended  beyond  the  base  of  the  mountain,  and  all  around 
there  was  a  cloudless  sky. 

257.  Relation  of  thunder  storms  to  cyclones.  The  preparation  of  synoptic 
weather  maps,  such  as  those  of  Figs.  64  and  67,  by  methods  more  fully 
explained  in  Chapter  XIII,  has  shown  that  well  developed  summer  thunder 
storms  occur  in  our  country  and  in  Europe  with  greatest  violence  and 
frequency  in  the  southern  or  southeastern  part  of  cyclonic  areas,  more  or  less 
independent  of  the  cyclonic  clouds  and  rain,  and  from  two  hundred  to  five  or 
six  hundred  miles  distant  from  the  center  of  low  pressure.  The  storms  are 
especially  strong  when  the  isobars  in  this  district  turn  outward  from  the 
center  in  a  pouch-like  curve.  When  the  cyclonic  area,  as  defined  by  its 
isobars,  takes  the  form  of  a  trough  or  "  V,"  elongated  to  the  south-southwest, 
as  in  Fig.  64,  thunder  storms  are  often  generated  near  or  east  of  the  axis  of  the 
trough,  as  if  along  the  line  of  more  rapidly  falling  temperature,  where  the 
southerly  winds  are  replaced  by  currents  from  the  west. 

The  general  features  of  this  important  relation  are  illustrated  in  Fig.  98, 
by  Hazen.  The  dotted  line 
crossing  the  Great  Lakes 
marks  the  track  of  a  cy- 
clonic center ;  its  place  at 
7  A.  M.  of  May  18,  19,  and 
20,  1884,  being  indicated  by 
the  figures  of  these  dates. 
A  series  of  curved  lines  is 
drawn  to  locate  the  suc- 
cessive positions  of  the 
belt  along  which  the  many 
thunder  storms  were  ob- 
served on  these  days  4  those 
of  May  18  being  broken 
lines ;  those  of  May  19 
being  full  lines.  The  posi- 
tion of  the  belt  at  certain  hours  is  indicated  by  numbers  at  the  end  of  the 
curved  lines,  counting  from  midnight  to  midnight.  It  thus  appears  that  while 


FIG.  98. 


258 


ELEMENTARY    METEOROLOGY. 


the  cyclonic  center  was  traversing  northern  Wisconsin,  the  belt  of  easy 
occurrence  of  thunder  storms  advanced  from  eastern  Iowa  and  northeastern 
Missouri  in  the  early  morning,  to  eastern  Indiana  and  central  Kentucky  in  the 
evening  ;  and  that  on  the  following  day,  while  the  cyclonic  center  crossed  from 
lower  Michigan  into  Canada,  the  belt  first  lay  obliquely  across  Illinois,  and 
then  rapidly  advanced  eastward,  increasing  greatly  in  length  as  if  influenced 
by  the  moister  southerly  winds  nearer  the  coast,  until  at  midnight  it  lay  along 
the  sea-board  from  Carolina  to  Connecticut.  During  all  this  time,  a  tolerably 
definite  relation  was  maintained  between  the  cyclonic  center  and  the  area  of 
thunder  storm  occurrence.  It  is  especially  noteworthy  that  on  May  19,  the 
advance  of  the  thunder  storm  belt  was  decidedly  more  rapid  than  the  pro- 
gression of  the  cyclonic  center ;  as  might  be  expected  from  the  dependence  of 
the  belt  on  the  winds  which  flow  around  the  cyclone,  and  which  therefore 
move  eastward  on  the  southern  side  of  the  cyclonic  area  faster  than  the  pro- 
gression of  the  center. 

It  is  believed  that  this  relation  of  thunder  storms  to  cyclones  gives  further 
evidence  of  their  dependence  on  atmospheric  instability ;  but  the  instability  is 
now  seen  to  be  determined  not  only  by  local  heat  due  to  sunshine  on  the  day 
of  occurrence,  but  also  by  the  importation  of  air  masses  in  the  cyclonic  circu- 
lation from  different  sources  and  with  different  temperatures.  The  following 
considerations  will  make  this  plain  :  Eastward  of  the  cyclonic  center,  its 
inflowing  winds  partake  of  the  nature  of  the  sirocco  ;  their  vertical  temperature 

gradient  is  represented  by  the  line  CDE,  Fig.  99  ; 
they  have  a  relatively  high  humidity  when  com- 
ing from  the  ocean,  as  has  been  explained  in 
Section  245.  Westward  from  the  cyclonic  center, 
the  winds  possess  something  of  the  character- 
istics of  the  cold  or  cool  wave,  with  a  vertical 
temperature  gradient  represented  by  the  line 
FGH,  and  a  relatively  low  humidity.  It  is  not 
possible  to  state  how  greatly  these  vertical 
temperature  gradients  depart  from  All,  the  mean 
vertical  temperature  gradient  for  the  region 
and  the  season;  but  it  is  believed  that  their 
departures  are  of  the  kind  here  indicated.  In  the. 
next  place,  numerous  studies  of  cyclonic  circu- 
lation have  shown  that  the  higher  currents  blow 
more  to  the  right  than  the  surface  winds.  It 
.p  therefore  follows  that  in  a  region  between  th'e 

district  of  the  sirocco  and  the  district  of  the  cool 

wave,  there  may  be  a  warm  surface  southerly  wind  overlain  by  a  cool  south- 
westerly wind  ;   or  a  warm  southwesterly  wind  overlain  by  a  cool  westerly 


LOCAL    STORMS.  259 

wind;  and  in  this  region,  the  Vertical  temperature  gradient  must  have  some- 
thing of  the  peculiar  form  indicated  in  the  curved  line  CDGH. 

It  is  at  present  impossible  to  give  any  definite  value  to  the  gradient, 
'  />'/'//,  at  various  altitudes  ;  but  the  considerations  presented  above  make  it 
extremely  probable  that  in  the  region  southeast  of  the  cyclonic  center,  the 
southerly  surface  winds  with  high  temperature  and  high  humidity  are  overlain 
at  a  considerable  altitude  by  westerly  winds  of  lower  temperature  and  lower 
humidity;  and  it  is  manifest  that  if  any  such  arrangement  exists,  the  oppor- 
tunity for  strong  convectional  overturning  is  thereby  greatly  increased  over  the 
opportunity  arising  only  from  the  local  heating  of  the  lower  air  near  the  ground. 

The  occurrence  of  the  greatest  number  of  thunder  storms  at  or  shortly  after 
the  hottest  hours  of  the  day  shows  that  the  provocation  to  overturning  that  is 
caused  by  local  and  diurnal  warming  of  the  lower  air  is  an  effective  assistance 
to  the  larger  instability  due  to  importation  of  unlike  air-masses  ;  the  vertical 
temperature  gradient  then  having  values  indicated  by  the  line  JD'GH.  On 
the  other  hand,  if  a  decided  instability  is  caused  by  importation,  thunder 
storms  may  continue  to  develop  after  nightfall,  when  the  cooling  of  the  lower 
air  has  changed  the  value  of  the  temperature  gradient  to  KD"GH.  It  is  how- 
ever manifest  that  theory  has  outstripped  observation  in  this  matter ;  and 
until  additional  facts  are  discovered  by  observation  on  mountains  or  in 
balloons,  the  discussion  of  the  special  value  of  vertical  temperature  gradients 
in  cyclonic  areas  need  not  be  pursued  further. 

There  is,  however,  another  way  in  which  the  cyclonic  importation  of  air 
masses  from  diverse  sources  and  with  different  temperatures  and  humidities 
has  been  thought  to  aid  in  the  production  of  thunder  storms,  particularly  in 
the  development  of  those  storms  that  advance  broadside  with  a  long  front. 
This  is  by  causing  a  lateral  instability  in  trough-like  cyclonic  areas,  across  the 
belt  where  the  cool  and  dry  westerly  winds  advance  close  to  the  area  occupied 
by  the  warm  and  moist  southerly  winds.  An  example  of  a  cyclonic  trough  of 
this  kind  is  shown  in  Fig.  64.  It  is  probable  that,  in  this  case  as  in  others, 
the  higher  members  of  the  westerly  currents  overrun  the  southerly  winds  for 
a  considerable  distance  eastward  of  the  line  that  separates  the  two  winds  at 
the  surface  of  the  earth  ;  for  isolated  local  storms  frequently  spring  up  within 
the  area  of  the  southerly  winds,  with  warm  air  extending  many  miles  west  of 
their  rain.  But  along  the  line  or  belt  where  the  cooler  wind  invades  the  area 
occupied  by  the  warmer  wind,  it  seems  as  if  the  latter  were  raised  by  the  under- 
running  of  the  former,  and  thus  caused  to  roll  over  on  itself ;  the  ascending 
portion  of  the  roll  being  recognized  by  the  formation  of  great  thunder  storm 
clouds.  Storms  of  this  kind  may  perhaps  be  distinguished  by  the  persistence 
of  the  change  of  temperature  and  winds  that  they  introduce.  Many  of  our 
larger  storms  may  be  referred  to  this  cause  ;  and  the  German  storm,  Fig.  93, 
is  thought  to  be  of  the  same  kind. 


260  ELEMENTARY   METEOROLOGY. 

It  follows  from  these  explanations  not  only  that  the  cyclonic  circulation 
may  afford  especially  favorable  opportunity  for  the  development  of  convtv- 
tional  thunder  storms  in  its  southern  or  eastern  quadrant  (in  our  hemisphere), 
but  also  that  so  good  an  opportunity  will  not  be  found  in  any  other  part  of  the 
cyclonic  area.  Nowhere  else  can  the  arrangement  of  the  cyclonic  indrafts 
produce  so  rapid  a  vertical  or  lateral  decrease  of  temperature  as  in  the  region 
to  the  southeast  of  the  center  of  low  pressure. 

The  strength  of  the  contrasts  of  temperature  in  cyclonic  winds  will  of 
course  depend  chiefly  on  the  atmospheric  contrasts  of  the  regions  whence  a 
cyclone  draws  its  supply  of  air.  In  the  torrid  zone,  where  the  distribution  of 
temperature  is  remarkably  equable,  no  definite  distribution  of  thunder  storms 
within  cyclonic  areas  has  been  detected.  On  the  great  southern  oceans,  where 
the  temperatures  are  comparatively  equable  and  relatively  low  in  summer, 
violent  thunder  storms  are  not  reported ;  although  when  the  numerous 
cyclonic  storms  of  the  southern  temperate  zone  cross  New  Zealand  in  summer, 
thunder  storms  occur  about  the  time  that  the  winds  change  from  northerly  to 
westerly  ;  that  is,  in  the  same  attitude  with  respect  to  the  cyclonic  center  as 
with  us.  There  is  good  reason  to  think  that  the  same  relation  obtains  in  the 
Argentine  Kepublic.  The  thunder  storms  of  both  Europe  and  the  United 
States  are  dominated  by  cyclonic  control ;  but  of  these  two  regions,  the  latter 
has  the  more  numerous  and  violent  storms,  because  of  the  stronger  contrasts 
of  its  southerly  and  westerly  winds.  It  should  not  therefore  be  thought  that 
cyclonic  action  always  develops  thunder  storms,  but  that  the  local  storms 
spring  up  in  the  larger  storms  chiefly  when  the  winds  of  the  latter  come  from 
regions  of  strongly  contrasted  temperature  and  moisture. 

It  must  furthermore  be  carefully  noted  that  the  cyclonic  control  of  thunder 
storms  is  not  by  any  means  absolute.  We  have  already  seen  that,  besides  the 
larger  thunder  storms  which  are  formed  along  the  axis  of  trough-like  cyclonic 
areas,  there  are  numerous  smaller  thunder  storms  distributed  somewhat  arbi- 
trarily in  the  area  of  the  warm  and  moist  southerly  winds.  Some  of  these  may 
be  independent  of  the  instability  that  is  supposed  to  result  from  overflow  by 
cooler  westerly  currents ;  they  may  depend  essentially  on  the  local  super- 
heating of  the  already  warm  and  moist  winds  as  they  advance  over  our 
summer  lands,  the  vertical  temperature  gradient  resembling  JD'E,  Fig.  99. 
Again,  it  is  well  known  that  thunder  storms  of  moderate  extent  but  often  of 
considerable  activity  spring  up  within  the  area  of  the  westerly  or  north- 
westerly winds,  southwest  or  west  of  the  cyclonic  center,  where  no  instability 
due  to  importation  of  unlike  air  masses  can  be  suspected.  These  storms  may 
be  best  explained  as  the  result  of  the  rapid  warming  and  moistening  of  the 
cool,  dry  westerly  current  as  it  flows  under  strong  sunshine  over  a  region 
watered  by  the  preceding  rain,  gaining  a  temperature  gradient  represented  by 
AGH,  Fig.  99.  Great  cumulus  clouds,  often  overflowing  at  the  top,  but 


LOCAL    STORMS.  261 

without  developing  fully  into  thunder  storms,  are  characteristic  of  this  region. 
A  storm  of  this  kind  occurred  in  southeastern  Massachusetts  on  July  17, 1889, 
while  its  parental  cyclone  was  far  down  the  St.  Lawrence  valley.  It  caused  a 
heavy  fall  of  hail,  by  which  several  valuable  cranberry  crops  were  laid  waste. 
Finally,  it  is  not  uncommon  to  encounter  thunder  storms  within  anticyclonic 
ureas  in  summer  time  ;  and  here  the  instability  on  which  the  overturning 
depends  must  be  referred  entirely  to  the  local  warming  and  moistening  of  the 
lower  air  by  intense  insolation  under  the  clear  anticyclonic  sky,  the  vertical 
temperature  gradient  taking  a  value  indicated  by  RD,  Fig.  86. 

258.  The  progression  of  thunder  storms.  The  general  advance  of  thunder 
storms  eastward  in  the  temperate  zone  and  westward  in  the  torrid  zone,  has 
already  been  stated.  It  appears  from  this  that,  like  smaller  clouds  on  the 
one  hand  and  like  larger  cyclonic  storms  on  the  other,  thunder  storms  advance 
chiefly  by  drifting  along  in  the  general  currents  of  the  atmosphere  in  which 
they  are  formed.  Their  relation  to  cyclonic  storms  finds  further  illustration 
in  this  respect,  for  they  move  in  directions  about  at  right  angles  to  the  line 
running  to  the  cyclonic  center ;  if  they  are  formed  in  the  southeastern 
quadrant,  they  generally  advance  towards  the  northeast;  if  in  the  south- 
western quadrant,  they  move  toward  the  southeast.  A  few  examples  have 
been  detected  in  Europe  north  of  cyclonic  centers,  and  moving  toward  the  west. 

The  velocity  of  progression  of  thunder  storms,  commonly  from  twenty  to 
fifty  miles  an  hour,  is  somewhat  greater  than  that  of  the  cyclonic  centers  that 
they  accompany.  Such  a  result  might  have  been  expected  because  the  local 
storms  are  borne  as  a  rule  in  the  winds  which  flow  around  the  larger  storm 
centers  in  the  direction  of  their  progression.  This  is  illustrated  in  Fig.  98. 
The  average  progression  of  the  cyclonic  center  was,  in  this  case,  twenty-one 
miles  an  hour ;  of  the  thunder  storms,  forty-one  miles  an  hour. 

The  course  followed  by  thunder  storms  within  the  cyclonic  area  gives 
strong  confirmation  to  the  suggestions  of  the  preceding  section  regarding  the 
overflow  of  westerly  currents  above  the  southerly  winds.  Even  the  isolated, 
thunder  storms  that  are  formed  southeast  of  the  cyclonic  center,  far  within 
the  area  of  the  southerly  winds,  move  eastward,  being  borne  along  by  a  high- 
level  current  in  that  direction. 

The  eastward  progression  of  thunder  storms  in  temperate  latitudes 
constantly  carries  them  from  the  warm  air  of  day-time  into  the  cooler  air  of 
the  night.  They  commonly  gather  strength  during  the  afternoon  and  weaken 
in  the  evening,  generally  ending  before  midnight  after  having  traversed  a  path 
of  two,  four  or  six  hundred  miles  in  length.  Occasionally  they  endure  longer, 
and  it  is  possible  that  some  which  have  been  observed  in  the  early  morning 
hours  have  lasted  over  from  the  previous  day  ;  but  there  is  no  decision  yet 
reached  on  this  matter. 


262  ELEMENTARY  METEOROLOGY. 

There  is  a  very  wide-spread  belief  that  thunder  storms  follow  valleys. 
Local  reports  mention  over  and  over  again  the  apparent  deviation  of  thunder 
storms  from  the  higher  ground  occupied  by  the  observer,  in  order  to  follow  a 
river  course  on  the  north  or  south.  It  is  possible  that  an  additional  strengtli 
may  be  given  to  cloud  growth  when  it  is  supplied  by  damp  air  from  low 
ground,  and  thunder  storms  may  in  this  way  grow  towards  valleys  and  weaken 
over  hills.  It  may  be  that  the  stronger  centers  of  action  in  a  long  thunder 
storm  front  seem  to  pass  more  commonly  to  one  side  than  directly  over  the 
observer,  and  that  this  is  then  interpreted  as  "  following  a  valley " ;  but  it 
may  be  safely  asserted  that  thunder  storms  as  a  whole  move  over  hills  and 
valleys  without  significant  deviation  from  their  direct  course.  The  example 
given  in  Fig.  92  of  the  thunder  squall  which  crossed  New  England  on  July  21, 
1885,  affords  a  striking  illustration  of  the  disregard  of  high  and  low  ground ; 
it  crossed  the  Hudson  valley,  the  Berkshire  plateau,  the  Connecticut  valley, 
and  the  highland  next  to  the  east  without  any  perceptible  regard  for  their 
strong  relief.  However  this  may  be  in  mountainous  regions,  it  is  clear  that 
the  surface  of  the  eastern  United  States  seldom  has  a  strong  enough  relief  to 
influence  the  course  of  thunder  storms.  They  move  in  air  currents  so  vast 
that  they  are  indifferent  to  the  moderate  inequalities  of  the  land. 

The  suggestion  made  above  as  to  the  aid  given  by  the  moister  air  of 
valleys  in  the  growth  of  thunder  storms  may  have  an  important  application 
regarding  the  place  of  their  more  frequent  beginning.  It  is  suspected  that  the 
storms  of  New  England  frequently  form  in  the  Hudson  valley  about  noon,  and 
therefore  arrive  in  New  England  in  the  late  afternoon  ;  Bavarian  thunder 
storms  have  similarly  been  referred  to  the  lowlands  of  the  middle  Khine  as  a 
place  of  beginning ;  but  further  observations  are  needed  on  this  point 

259.  The  thunder  squall.  The  violent  outrushing  squall  of  cool  wind  that 
commonly  precedes  our  stronger  thunder  storms  has  been  regarded  by  some 
observers  as  contradicting  the  belief  in  the  convectional  character  of  the  storm 
as  a  whole  j  but  when  the  subordinate  dimensions  and  position  of  the  squall  with 
reference  to  the  other  parts  of  the  storm  are  properly  perceived,  this  contra- 
diction entirely  disappears.  The  squall  does  not  reach  to  a  great  height  above 
the  surface  of  the  earth ;  its  upper  limit  must  be  below  the  dark  lower  front 
edge  of  the  storm  cloud —  perhaps  half  a  mile  or  less  above  the  ground  —  for 
there  the  wisps  of  the  cloud  front  demonstrate  an  inflow  with  respect  to  the 
storm  in  the  most  unmistakable  manner.  The  squall  does  not  extend  far 
beyond  the  front  of  the  main  cloud  mass  ;  further  in  advance  of  the  clouds,  the 
warm  surface  wind  often  blows  obliquely  or  directly  towards  the  approaching 
storm.  Fig.  100  represents  the  lower  front  of  a  thunder  storm,  with  the 
squall  below  it ;  showing  the  space  bounded  by  the  letters  dbqs,  Fig.  88,  on  a 
larger  scale.  The  dimensions  of  the  squall  are  therefore  relatively  small, 


LOCAL,    STORMS. 


263 


though  its  violence  is  excessive.  It  is  a  forward  outflow  from  the  bottom  of 
the  storm,  but  its  occurrence  by  no  means  indicates  that  the  whole  mass  of 
air  in  the  storm  is  descending. 

The  cause  of  the  squall  has  been  sought  in  the  downward  brushing  of  the 
air  by  the  falling  rain ;  but  the  squall  sometimes  occurs  under  storm  clouds 
from  which  no  rain  falls.  It  has  been  ascribed  to  the  descent  of  air  that  lias 
cooled  under  the  shadow  of  the  cloud  ;  but  it  occurs  at  night  as  well  as  by  day. 
It  is  best  explained,  following  a  suggestion  by  Ferrel,  as  the  result  of  a  down- 
ward reaction  from  the  upward  expansion  of  the  great  mass  of  air  involved  in 
the  storm  cloud.  It  is  therefore  analogous  to  the  seaward  reaction  of  the 


X 


FIG.   100. 

expanding  air  over  a  warm  land  area,  by  which  the  incoming  of  the  morning 
sea  breeze  is  delayed  (Sect.  161).  It  may  even  be  compared  to  the  "kick"  of 
a  gun,  and  with  more  justice  than  at  first  appears.  In  the  convectional 
overturning  of  cloudless  air,  the  expansion  of  that  which  ascends  is  practically 
counter-balanced  by  the  compression  of  that  which  descends  ;  and  hence  there 
is  no  considerable  change  of  volume  as  a  result  of  the  overturning.  But  in 
convectional  action  where  clouds  are  formed,  the  volume  of  the  air  concerned 
in  the  overturning  is  actually  greater  after  than  before  the  change.  The 
descending  air  is  compressed  at  the  usual  rate  ;  but  the  ascending  cloudy  air 
expands  more  than  in  the  first  case,  because  its  cooling  is  retarded  by  the 
liberation  of  latent  heat  from  its  condensed  vapor.  To  gain  room  for  this 
increase  of  volume,  a  considerable  mass  of  surrounding  or  overlying  air  must 
be  pushed  away  by  the  ascending  and  expanding  air ;  and  in  reacting  on  the 
ground  it  presses  downward  with  more  than  its  weight  ;  thus  causing  at  once 


264  ELEMENTARY    METEOROLOGY. 

the  slight  rise  in  the  barometer  as  the  storm  comes  on,  and  the  outrushiug 
squall  wind. 

The  development  of  the  squall  chiefly  along  the  front  of  the  storm,  its  less 
violence  on  either  side,  and  its  absence  in  the  rear  seem  to  be  chiefly  a  result 
of  the  forward  motion  of  the  storm  as  a  whole.  The  entire  cloud  mass  floats 
forward  at  a  rate  of  from  twenty  to  forty  miles  an  hour.  Part  of  the  air 
is  pushed  outward  at  the  bottom.  At  the  rear  of  the  storm,  the  outward  push 
is  neutralized  by  the  forward  drift  ;  moreover,  the  inflow  that  supplies  the 
cloud  is  here  relatively  weak.  In  front  of  the  storm,  the  two  velocities  of 
progression  and  expansion  are  combined,  and  the  outflow  thus  becomes  a 
destructive  squall.  This  explanation  is  confirmed  by  the  occasional  occurrence 
of  stationary  thunder  storms,  in  which  the  squall  is  felt  with  about  equal 
violence  on  all  sides  of  the  base  of  the  cloud. 

A  gray  roll  of  cloud  is  generally  observed  at  a  little  distance  back  of  the 
dark  lower  cloud  front ;  if  carefully  watched,  it  may  be  seen  to  turn  slowly 
between  the  inflow  above  and  the  outflow  below.  Its  presence  seems  to  depend 
on  an  eddy  caused  by  the  squall,  and  it  is  therefore  called  the  squall  cloud,  as 
in  Fig.  100.  A  little  way  behind  it,  ragged  cloud  margins  may  be  seen,  from 
which  the  wisps  settle  down  and  dissolve  away,  as  if  brushed  down  by  the 
squall  wind. 

260.  Nocturnal  thunder  storms.  A  peculiar  exception  to  the  general  rule 
of  the  occurrence  of  thunder  storms  in  the  hotter  seasons  and  hours  is  found 
over  the  North  Atlantic  ocean  in  the  middle  and  higher  latitudes  —  and 
probably  over  other  oceans  in  similar  latitudes  —  and  on  the  bordering  coasts. 
In  Iceland,  for  example,  of  twenty-three  thunder  storms  recorded  there  in 
fourteen  years,  twenty-two  were  noted  in  the  colder  months  and  twenty  were 
heard  between  sunset  and  sunrise.  In  Norway,  most  of  the  thunder  storms 
occur  on  summer  afternoons ;  but  the  smaller  number  observed  iri  winter 
along  the  coast  are  more  common  at  night  than  by  day.  The  same  rule  holds 
good  for  western  Scotland.  Over  the  ocean,  the  excess  at  night  seems  to  hold 
good  ;  but  the  proportion  during  different  seasons  is  not  well  made  out.  On 
our  New  England  coast,  winter  thunder  storms  are  relatively  rare ;  but  when 
they  occur  they  are  generally  nocturnal. 

The  clear  indication  of  convectional  action  in  summer  thunder  storms 
on  land  leads  to  the  belief  that  those  of  winter  at  sea  should  be  convectioual 
also;  but  in  that  case,  their  occurrence  at  night  remains  to  be  explained.  It 
has  been  suggested  that  the  reason  for  this  may  be  found  in  the  development 
of  instability  at  such  times  by  the  excessive  cooling  of  the  upper  layers  of 
air  by  radiation  from  the  lofty  cyclonic  cloud  sheets  in  which  winter  marine 
thunder  storms  are  developed ;  while  the  lower  layers  of  air,  on  or  near  the 
ocean  surface,  are  maintained  at  a  comparatively  high  temperature.  This 


LOCAL    STORMS.  265 

might  be  reasonably  expected  along  the  path  of  the  Gulf  Stream,  or  in  warm 
winds  derived  from  it.  The  resemblance  of  such  a  process  to  the  development 
of  the  bora  (Sect.  247)  should  be  noted. 

261.  Atmospheric  electricity.  It  is  necessary  to  make  a  brief  digression 
lu'ie  in  order  to  introduce  some  account  of  atmospheric  electricity  in  general 
before  speaking  of  its  more  intense  manifestations  in  thunder  storms.  The 
air  nearly  always  possesses  a  slight  positive  electric  charge,  compared  with 
the  earth.  The  source  of  this  charge  has  been  variously  ascribed  to  the  forma- 
tion of  vapor  from  water  surfaces  —  but  it  is  doubtful  if  the  process  of  quiet 
evaporation  suffices  to  account  for  it ;  to  the  friction  of  air  on  bodies  that 
resist  its  motion,  as  dry  air  in  dusty  whirlwinds  —  but  the  quantity  of 
electricity  thus  produced  is  small ;  to  the  friction  of  water  particles  and  dry 
snow-flakes  in  the  upper  part  of  agitated  clouds  —  but  this  process  seems  too 
exceptional  to  serve  as  the  cause  of  so  general  a  result.  Further  investigation 
is  needed  on  this  subject. 

In  studying  the  electrical  condition  of  the  atmosphere,  it  is  found  that  the 
positive  potential  increases  with  altitude  above  the  ground.  It  is  subject  to  a 
slight,  double  diurnal  variation,  best  observed  in  settled  clear  weather,  having 
maxima  in  the  morning  and  evening,  and  minima  before  sunrise  and  in  the 
afternoon.  It  has  also  an  irregular  variation,  accompanying  changes  in  the 
weather  ;  the  positive  charge  being  greatest  under  a  clear  sky,  especially  in 
the  dry,  cold  air  of  winter  anticyclones.  In  the  presence  of  clouds,  the  charge 
varies  greatly  ;  sometimes  becoming  negative,  or  frequently  changing  its  sign, 
especially  in  thunder  storms,  when  its  fluctuations  are  great  and  rapid.  Its 
variations  are  therefore  intimately  connected  with  the  quantity  and  condition 
of  atmospheric  vapor. 

As  a  rule,  the  electricity  of  the  atmosphere  does  not  suffice  to  produce 
attractive  or  repulsive  forces  of  sufficient  amount  to  cause  significant  move- 
ments of  the  air.  Exceptions  to  this  statement  may,  perhaps,  be  found  in 
thunder  storms,  but  even  there  the  general  movement  of  the  air  seems  to 
follow  a  convectional  and  not  an  electrical  cause,  as  has  already  been  described. 
Although  many  attempts  have  been  made  to  explain  local  storms  by  electrical 
action,  it  has  not  yet  been  shown  that  the  observed  electrical  forces  nearly 
suffice  to  account  for  the  results  witnessed  ;  nor  has  there  been  offered  in  such 
theories  any  reasonable  and  sufficient  cause  for  the  local  development  of 
electricity  even  in  its  observed  insufficient  amounts.  It  is  manifest  that  a 
tenable  electrical  theory  of  storms  must  present  a  valid  cause  for  the  concen- 
tration of  the  electricity  by  which  storms  are  to  be  produced ;  and  that  a 
theory  is  doubly  at  fault  in  calling  upon  an  unexplained  charge  of  electricity 
to  produce  results  not  accordant  in  quantity  and  quality  with  the  observed 
ettects  of  electric  action.  For  example,  in  the  working  of  a  glass  plate 


266  ELEMENTARY    METEOROLOGY. 

electrical  machine,  a  sufficient  force  must  be  applied  to  turn  the  plate  before 
the  sparks  appear  ;  so  in  the  atmosphere,  sufficient  forces  must  be  in  operation 
to  produce  the  intense  charge  that  results  in  lightning  flashes ;  but  in  both 
cases,  the  electric  action  is  essentially  the  effect  and  not  the  cause  of  the  other 
motions.  The  working  forces  of  thunder  storms  are  best  accounted  for  by  the 
convectional  processes  already  described  ;  and  it  is  therefore  more  reasonable 
to  place  the  lightning  along  with  the  clouds  and  rain,  as  secondary  effects  of 
the  convectional  storm,  than  to  regard  any  one  of  them,  itself  not  accounted 
for,  yet  serving  as  the  cause  of  the  others.  While  atmospheric  electricity  is 
an  important  branch  of  terrestrial  physics,  and  while  in  thunder  storms  it 
attains  an  extraordinary  display  as  an  effect  of  the  storm,  it  does  not  generally 
appear  to  be  a  factor  of  much  importance  in  the  processes  of  meteorology. 
In  studying  the  general  and  local  movements  of  the  atmosphere,  we  are 
hardly  more  concerned  with  atmospheric  electricity  than  with  atmospheric 
composition. 

262.  Lightning.  The  identity  of  lightning  with  artificial  electric  sparks 
was  suggested  by  Franklin  shortly  before  the  middle  of  the  eighteenth  century 
and  demonstrated  by  experiment  in  France  shortly  after,  as  well  as  by  Franklin 
himself  in  his  famous  experiment  with  a  kite  in  Philadelphia  in  1752. 

When  the  growth  of  thunder  storms  is  observed  from  their  first  appearance 
as  cumulus  clouds,  it  may  be  seen  that  lightning  flashes  first  occur  at  about 
the  time  when  the  cirro-stratus  sheet  begins  to  spread  out  from  the  top  of  the 
cloud  ;  and  that  from  this  time  on  the  electric  activity  of  the  storm  increases 
with  its  further  cloud  growth.  The  quantity  of  electricity  in  the  cloud  is 
continually  increased  by  the  inflow  of  moist  air  at  its  lower  margin.  It  is 
believed  that  the  electric  potential  is  increased  by  the  aggregation  of  many 
extremely  small  cloud  particles  into  a  smaller  number  of  larger  droplets,  and 
finally  into  rain-drops  ;  for  the  initial  charge  resides  on  the  surface  of  each 
minutest  particle,  and  with  the  successive  aggregation  of  particles,  the  quantity 
of  electricity  increases  faster  than  the  surface  area  of  the  droplet.  Thus  with 
the  growth  of  the  cloud,  there  is  both  increased  potential  and  increased  quantity 
of  electricity.  It  is  probable  that  this  process  goes  on  in  all  cases  of  cloud 
formation  ;  but  that  a  potential  high  enough  to  cause  lightning  flashes  is  pro- 
duced only  when  the  cloud  growth  is  rapidly  and  continually  augmented  by 
inflow  of  moist  air  at  the  base.  Then  the  cloud  droplets,  suspended  in  the 
ascensional  current  of  the  cloud,  gain  a  continual  increase  in  both  the  quantity 
and  potential  of  their  electric  charge,  until  a  flash  occurs.  In  our  ordinary 
cyclonic  storms,  the  cloud  growth  is  gradual  and  the  vertical  component  of 
movement  is  much  slower  than  in  thunder  storms  ;  hence  there  is  less  oppor- 
tunity for  the  increase  in  the  quantity  of  the  charge  by  the  process  above 
suggested,  and  flashes  are  relatively  rare.  In  tropical  cyclones,  where  the 


LOCAL    STORMS.  267 

convectional  process  is  much  more  active  than  in  our  latitudes,  lightning  is 
often  frequent  and  vivid. 

It  is  probable  that  when  the  different  parts  of  a  thunder  cloud  are  thus 
variously  charged,  the  lightning  flashes  depend  for  their  opportunity  in  great 
measure  on  the  movement  of  cloud  masses  and  rainfalls  within  the  storm.  In 
this  way,  volumes  of  cloudy  or  rainy  air  of  different  charges  may  move  about, 
rising  or  falling  until  they  come  within  striking  distance  of  one  another,  or  of 
the  earth.  It  is  also  suggested  that  the  discharge  of  a  flash  may  allow  the 
union  of  many  small  droplets  that  were  before  held  apart  by  electric  repulsion, 
and  thus  locally  promote  the  fall  of  rain.  The  heavier  fall  of  rain  that  often 
reaches  the  ground  shortly  after  a  brilliant  flash  of  lightning  may  perhaps  be 
explained  by  either  one  of  these  theories  ;  being  regarded  as  the  cause  of  the 
flash  in  the  first,  and  the  effect  of  the  flash  in  the  second  theory.  It  is  also 
possible  that  an  intensely  charged  fall  of  rain  may  charge  the  particles  near 
its  path  by  induction,  causing  them  to  attract  one  another  and  thus  promoting 
their  coalescence  and  an  increase  in  their  potential. 

Lightning  flashes  consist  of  several  extremely  brief  sparks,  each  one 
almost  instantaneous,  separated  from  one  another  by  small  fractions  of  a 
second ;  hence  the  vibrating  or  flickering  appearance  of  lightning  often 
noticed.  The  composite  nature  of  a  flash  is  easily  shown  by  looking  at  a 
thunder  storm  through  a  narrow  slit  in  a  rapidly-revolving  disc ;  the  slit 
being  seen  in  several  apparently  stationary  positions  at  each  flash.  The 
successive  sparks  are  sometimes  separately  visible  to  the  unaided  eye.  Each 
spark  is  believed  to  consist  of  many  excessively  rapid  electric  undulations, 
which  cease  when  their  energy  is  exhausted  in  overcoming  the  resistances  on 
their  path. 

By  far  the  greater  number  of  flashes  pass  from  one  cloud  to  another.  It 
is  probable  that  they  then  begin  and  end  in  innumerable  fine  branches  on 
countless  cloud  particles,  and  that  the  branches  unite  in  the  space  between  the 
clouds  to  form  the  single  or  composite  trunk  flash  that  we  commonly  observe. 
The  brighter  terminal  branches  are  sometimes  visible  to  the  eye ;  but  they  are 
better  perceived  on  a  sensitive  photographic  plate,  exposed  at  night  in  the 
direction  of  an  active  storm.  Sometimes  the  flash  passes  from  a  cloud  out  to 
the  open  air,  gradually  dissipating  itself.  The  length  of  flashes  may  reach 
several  miles  ;  but  this  has  seldom  been  well  determined.  When  a  flash  strikes 
the  earth,  it  commonly  selects  some  elevated  point,  such  as  a  tree  or  church 
spire.  The  greater  part  of  the  discharge  may  then  enter  at  a  single  point ; 
but  branches  are  often  observed  diverging  towards  various  objects  on  the 
ground,  and  sometimes  in  great  number,  like  the  roots  of  a  tree,  or  the 
fine  endings  of  a  nerve.  A  flash  does  not  follow  an  angular  zig-zag  line, 
as  it  has  been  commonly  represented  in  pictures ;  photographs  show  it  to 
run  in  a  sinuous  path,  somewhat  like  a  river  course.  An  apparently  looped 


268  ELEMENTARY  METEOROLOGY. 

or  recrossing  flash  may  be  produced  by  the  foreshortening  of  a  twisted  or 
helical  path. 

Lightning  may  be  seen  over  great  distances.  Thunder  storms  in  northern 
Italy  have  been  witnessed  in  southern  Germany,  over  the  intervening  Alps. 
The  illumination  of  distant  thunder  clouds  by  inaudible  lightning  flashes  is 
commonly  called  heat  lightning;  but  this  is  not  shown  to  differ  from  ordinary 
flashes,  except  in  its  distance  from  the  observer.  The  occurrence  of  sheet 
lightning,  reported  by  various  observers,  may  be  generally  explained  as 
the  illumination  of  clouds  by  inaudible  flashes ;  but  it  is  possible  that  dis- 
charges in  the  thin  upper  air  may  be  quiet  and  sheet-like,  rather  than  noisy 
and  disruptive. 

Discharges  of  atmospheric  electricity  occasionally  take  the  form  of  globe 
lightning,  having  the  appearance  of  luminous  balls,  seeming  to  be  a  foot  or  so 
in  diameter,  moving  at  a  moderate  velocity  and  passing  about  among  objects 
near  the  ground ;  remaining  visible  a  number  of  seconds,  and  commonly 
disappearing  with  an  explosion.  No  satisfactory  explanation  has  been  offered 
for  this  curious  phenomenon  :  careful  observation  should  be  made  of  it.  Weak 
brush-like  discharges,  known  as  St.  Elmo's  fire,  are  sometimes  observed  during 
stormy  weather  on  trees  and  house  tops,  or  on  the  yards  and  masts  of  vessels 
at  sea.  A  similar  appearance  has  often  been  noted  on  mountain  summits 
within  storm  clouds  ;  all  pointed  objects  being  surmounted  by  a  bluish  flame- 
like  light,  from  which  a  buzzing  or  crackling  sound  is  emitted.  The  fingers  of 
an  observer  may  thus  discharge  an  electric  stream  into  the  air. 

263.  Thunder.  The  brilliancy  of  lightning  is  due  to  the  excessive 
vibration  of  the  luminiferous  ether  caused  by  the  flash ;  the  deafening  sound 
of  the  thunder  results  from  violent  vibrations  excited  at  the  same  time  in  the 
air.  The  sudden  heating  and  electric  disturbance  along  the  path  of  the  flash 
have  much  the  same  effect  in  producing  sound  as  the  firing  of  an  explosive 
substance.  When  a  flash  occurs  near  the  observer,  the  sharp  crackling  reports 
first  heard  come  from  the  smaller  branches  that  are  nearer  than  the  trunk ; 
the  heavy  crash  immediately  following  comes  from  the  nearer  part  of  the  trunk 
flas li ;  and  the  rolling  thunder  that  then  succeeds  comes  from  the  more  distant 
part  of  the  trunk,  as  well  as  from  reverberation  among  the  clouds.  The  rolling 
is  greatly  intensified  among  lofty  mountains. 

As  sound  travels  through  the  air  with  a  velocity  of  about  1,100  feet  a 
second,  the  distance  of  a  flash  in  miles  is  roughly  equal  to  one-fifth  of  the 
number  of  seconds  —  or  pulse  beats  —  counted  between  the  flash  and  its 
thunder.  It  is  seldom  that  a  longer  interval  than  seventy  or  eighty  seconds 
elapses  between  a  flash  and  its  sound.  The  velocity  of  progression  of  a 
thunder  storm  may  be  estimated  by  recording  the  time  at  which  successive 
flashes  appear,  and  determining  the  distance  of  each  one.  In  doing  this;  it 


LOCAL    STORMS.  269 

will  be  noticed  that  the  nearest  flashes  generally  occur  just  after  the  beginning 
of  the  heavy  rain ;  that  is,  near  the  front  of  the  storm,  where  the  inflow  of 
moist  air  and  the  cloudy  condensation  of  its  vapor  are  most  active. 

264.  Lightning  strokes  and  lightning  rods.1  When  lightning  strikes  the 
earth,  it  sometimes  fuses  the  sand  along  its  path,  forming  vitrified  tubes  or 
fulgurites,  having  a  depth  of  several  feet  below  the  surface.  A  flash  may  pass 
along  beneath  the  surface  at  a  slight  depth,  turning  up  a  furrow  of  earth, 
probably  by  the  sudden  vaporizing  of  the  moisture  that  it  encounters.  In 
striking  trees,  the  bark  may  be  split  off,  or  the  trunk  shattered ;  but  if  the 
bark  is  smooth  and  well  wet  by  rain,  little  injury  may  be  done.  When  an 
unprotected  house  is  struck,  its  walls  are  more  or  less  fractured,  and  if  built 
of  wood,  it  may  be  set  on  fire.  There  is  no  truth  in  the  saying  that  lightning 
never  strikes  twice  in  the  same  place. 

It  is  reported  that  in  the  six  years,  1885-1890,  there  were  2,223  buildings 
set  on  fire  by  lightning  in  this  country,  or  1.3  per  cent  of  the  total  number  of 
fires ;  the  loss  thus  occasioned  being  $3,386,826,  or  1.25  per  cent  of  the  total 
fire  loss.  It  is  further  reported  that  205  persons  were  killed  by  lightning  in 
this  country  in  1891,  and  292  in  1892.  Over  95  per  cent  of  these  casualties 
occur  between  April  and  September,  or  within  half  of  the  year.  When  a 
person  is  apparently  killed  by  lightning,  it  may  be  that  no  permanent  injury  is 
effected,  but  that  respiration  is  stopped  by  a  temporary  paralysis.  Efforts  to 
restore  respiration  should  be  continued  for  an  hour. 

All  these  disruptive  and  harmful  effects  are  caused  by  the  expenditure  of 
the  energy  of  the  flash  on  poor  or  insufficient  conductors  that  happen  to  lie  in 
its  path.  It  is  therefore  desirable  to  protect  buildings  by  providing  them  with 
conductors  or  lightning  rods,  in  which  the  undulating  sparks  of  the  flash  may 
pass  easily,  leaving  them  to  exhaust  themselves  elsewhere.  Lightning  conduc- 
tors are  best  made  in  the  form  of  continuous  copper  rods  or  tapes  terminating 
upwards  in  one  or  several  sharp  points,  ten  or  fifteen  feet  above  the  building ; 
and  extending  downward  beneath  the  building  deep  enough  to  be  in  permanently 
wet  ground.  The  last  point  is  extremely  important ;  many  rods  fail  to  protect 
buildings  by  reason  of  the  neglect  of  this  essential  in  their  construction. 

It  is  found,  however,  that  buildings  are  sometimes  injured  even  when 
provided  with  good  rods.  The  cause  of  this  difficulty  seems  to  lie  in  the 
variation  of  the  intensity  of  flashes.  Moderate  discharges  can  be  safely 
disposed  of  by  ordinary  rods  ;  but  excessive  discharges  overwhelm  the 
conductors  or  their  connection  with  the  ground  ;  and  then  destructive  branching 
may  occur  through  the  building.  A  number  of  tall  trees  near  a  house  probably 
afford  better  protection  than  most  lightning  rods. 

1  See  "Lightning  Conductors  and  Lightning  Guards,"  by  O.  J.  Lodge. 


270  ELEMENTARY  METEOROLOGY. 

265.  The  aurora  borealis,  often  called  northern  lights,  is  an  illumination 
of  the  atmosphere  in  arches,  streamers,  patches  or  sheets  of  whitish,  yellow, 
green  or  red  light,  caused  by  diffuse  electrical  discharges  chiefly  in  the  thin 
upper  air.  It  is  occasionally  bright  enough  to  be  seen  in  the  day-time. 
Although  irrelevant  to  the  chief  subject  of  this  chapter,  some  account  of  it  is 
conveniently  introduced  in  connection  with  atmospheric  electricity,  of  which 
it  is  a  peculiar  manifestation. 

The  aurora  is  most  common  and  brilliant  in  relatively  high  latitudes, 
along  a  belt  that  follows  near  the  shores  of  the  Arctic  ocean  from  the  North 
Cape  of  Europe  eastward  to  Point  Barrow  in  northwestern  America  ;  thence 
somewhat  southward,  so  as  to  pass  through  Hudson  Bay  in  latitude  60°,  and 
a  little  south  of  Greenland  ;  and  obliquely  northward  again  between  Iceland  and 
the  Faroe.  On  either  side  of  this  belt,  the  aurora  is  less  common  ;  and  in  the 
torrid  zone,  it  is  rarely  observed.  The  aurora  australis  is  seen  in  high  southern 
latitudes,  but  its  distribution  has  been  little  studied  for  want  of  observations. 

As  commonly  seen  in  our  latitudes,  the  aurora  begins  with  the  formation 
of  an  arch,  more  or  less  complete,  with  its  apex  in  the  magnetic  meridian  ; 
the  lower  side  of  the  arch  being  better  denned  than  the  upper,  and  the  sky 
beneath  seeming  darkened  by  contrast  ;  but  stars  are  visible  there  as  well  as 
through  the  aurora  itself.  The  angular  altitude  at  which  the  arch  forms  is 
greater  in  higher  latitudes,  until  in  the  belt  of  greatest  frequency  the  arch 
crosses  the  zenith,  stretching  at  right  angles  to  the  magnetic  meridian. 
Further  towards  the  pole,  it  is  seen  to  the  south  of  the  observer.  The  arch  is 
sometimes  evenly  illuminated  ;  sometimes  convoluted  like  a  folded  curtain  ; 
but  it  is  more  commonly  banded  with  rays  nearly  at  right  angles  to  its  curve. 
In  the  greater  displays,  the  rays  are  prolonged  upwards  into  streamers,  which 
seem  to  converge  in  a  corona  high  in  the  sky,  nearly  in  the  direction  indicated 
by  the  south  end  of  a  magnetic  dipping  needle.  The  light  of  the  streamers 
often  flashes  rapidly,  whence  the  name,  "  merry  dancers,"  sometimes  given  to 
them.  After  its  formation,  the  arch  frequently  moves  slowly  away  from  the 
direction  of  the  belt  of  greatest  frequency ;  that  is,  towards  the  equator  in  our 
latitudes,  and  towards  the  pole  in  the  Arctic  regions.  As  it  moves,  it  has  been 
noted  that  the  apparent  breadth  of  the  arch  diminishes  on  approaching  the 
coronal  point ;  that  the  streamers  make  a  more  and  more  acute  angle  with  the 
arch  until  they  coalesce  with  it  when  it  passes  through  the  coronal  point  ; 
that  the  brightness  of  the  arch  is  greatest  when  it  is  narrowest ;  and  that  on 
passing  the  coronal  point,  these  changes  proceed  in  the  reverse  order.  It  is 
concluded  from  these  facts  that  the  arch  is  like  a  sheet,  hanging  nearly 
vertical ;  and  that  the  rays  or  streamers  are  nearly  parallel,  their  apparent 
divergence  from  the  coronal  point  being  an  effect  of  perspective,  like  that  by 
which  the  beams  of  the  sun  shining  between  clouds,  or  the  paths  of  a  group  of 
shooting  stars  are  given  an  appearance  of  divergence. 


LOCAL    STOKMS. 

It  is  believed  that  the  auroral  arch  is  a  more  or  less  extended  arc  of  a 
circle  whose  plane  is  at  right  angles  to,  and  whose  center  lies  in  the  magnetic 
axis  of  the  earth.  Accepting  this  conclusion,  the  height  of  the  arch  has  been 
found  to  vary  between  33  and  281  miles,  averaging  130  miles  above  the  earth's 
surface.  Its  streamers  seem  to  extend  to  even  greater  heights.  On  the  other 
hand,  faint  rays  have  been  reported  as  being  visible  between  an  observer  and  a 
neighboring  mountain  or  a  low  cloud.  Systematic  observations  by  numerous 
observers  are  needed  on  this  point. 

Besides  the  geometrical  forms  of  arches  and  streamers,  the  aurora  has 
many  irregular  appearances,  of  which  no  account  can  be  given  here.  Its 
duration  varies  greatly,  from  a  faint  light  for  a  few  minutes,  to  brilliant 
displays  lasting  many  hours,  or  perhaps  enduring  over  several  days.  These 
greater  displays  have  been  witnessed  over  large  parts  of  the  earth  ;  while  the 
ordinary  lights  are  relatively  local.  The  relation  of  auroral  displays  to 
atmospheric  conditions,  such  as  control  weather  changes,  is  not  well  made 
out,  although  they  are  commonly  associated  with  fine  clear  skies.  Certain 
Arctic  observations,  during  long  polar  nights,  indicate  that  the  aurora  is  more 
frequent  in  the  nocturnal  than  in  the  diurnal  hours.  It  exhibits  a  double 
annual  period,  being  more  common  in  March  and  October,  and  less  common  in 
January  and  June.  It  also  has  a  period  of  greater  frequency  about  every 
eleven  years  ;  numerous  aurorae  corresponding  with  numerous  sun-spots,  and 
with  the  stronger  disturbances  of  the  magnetic  needle  ;  thus  indicating  some 
association  of  terrestrial  magnetism  and  auroral  displays  with  solar  action. 
There  is  also  a  longer  variation  in  the  frequency  of  aurorae  ;  they  were 
relatively  rare  from  1795  to  1823,  and  relatively  frequent  about  1780-90, 
1850  and  1870.  While  those  various  relations,  both  of  place  and  time,  leave 
no  doubt  that  the  aurora  is  an  electric  discharge  chiefly  in  the  upper  air,  and 
dependent  in  some  way  on  the  magnetic  conditions  of  the  earth  and  sun,  the 
full  nature  of  its  controls  and  processes  are  by  no  means  understood. 

TORNADOES  AND  WATERSPOUTS. 

266.  Tornadoes  are  local  whirlwinds  of  great  energy,  generally  formed 
within  thunder  storms.  Their  most  invariable  feature  is  a  funnel-shaped 
cloud  that  hangs  from  the  bottom  of  the  greater  thunder  cloud  mass  above. 
The  funnel  is  created  around  the  axis  of  a  violent  ascending  vortex  of  whirling 
winds  ;  its  diameter  may  reach  a  few  hundred  feet,  being  much  greater  above 
than  below ;  while  the  destructive  winds  around  it  cover  a  somewhat  greater 
space.  The  whirling  funnel  advances  generally  eastward  or  northeastward, 
at  a  rate  of  twenty,  thirty  or  forty  miles  an  hour,  with  a  deafening  roaring 
noise,  destroying  everything  that  its  winds  fall  on  in  their  rapid  passage. 
The  activity  of  a  single  tornado  may  continue  for  half  an  hour  or  an  hour, 


272  ELEMENTARY  METEOROLOGY. 

while  it  lays  waste  a  path  five  to  twenty  or  more  miles  long,  and  commonly  less 
than  a  quarter  of  a  mile  wide.1  Waterspouts  at  sea  are  of  essentially  the  same 
nature  as  tornadoes  on  land ;  but  their  association  with  thunder  storms  is  less 
marked. 

Although  careful  observations  of  passing  tornadoes  are  seldom  made,  on 
account  of  the  dread  they  naturally  inspire,  there  are  many  accounts  of  them 
from  observers  who  were  at  a  safe  distance  on  one  side  of  their  track ;  and 
from  these  we  have  repeated  accounts  of  the  whirling,  writhing  funnel  cloud, 
which  constitutes  the  visible  part  of  the  ascending  vortex.  An  old  account  of 
a  " spout "  in  England  in  1587  is  as  follows  :  —  "The  wind  thus  blowing  soon 
created  a  great  vortex,  giration  and  whirl  among  the  clouds,  the  center  of 
which  ever  now  and  then  dropt  down  in  the  shape  of  a  thick  long  black  pipe, 
commonly  called  a  spout ;  in  which  I  could  plainly  and  most  distinctly  behold 
a  motion,  like  that  of  a  screw,  continually  drawing  upwards  and  screwing  up 
(as  it  were)  whatever  it  touched."  A  tornado  in  central  Massachusetts  in 
1760  was  thus  described:  —  "At  Leicester,  several  people  of  credit  say  that 
about  five  o'clock  the  sky  looked  strangely ;  that  clouds  from  the  southwest 
and  northwest  seemed  to  rush  together  very  swiftly,  and  immediately  upon 
their  meeting,  commenced  a  circular  motion  ;  presently  after  which  a  terrible 
noise  was  heard.  The  whirlwind  cut  its  path  through  the  trees,  and  after 
having  passed  over  some  clear  land,  it  came  to  the  dwelling  of  one  David 
Lynde,  the  only  one  which  stood  in  its  way ;  upon  this  it  fell  with  the  utmost 
fury  and  in  a  moment  effected  its  complete  destruction."  The  following 
extract  describes  a  tornado  that  was  carefully  observed  in  northern  Alabama 
in  1867.  When  first  seen,  the  funnel  was  about  five  miles  away,  and  judging 
from  its  angular  altitude  above  the  horizon,  its  height  was  estimated  at  4,500 
feet ;  coming  nearer,  it  passed  about  nine  hundred  feet  south  of  the  observer, 
when  the  gyratory  motion  of  the  cloud  was  distinctly  visible.  Many  small 
objects  gathered  from  the  ground  were  perceived  flying  around  the  summit  of 
the  column  like  a  great  flock  of  birds  ;  and  a  pine  tree,  afterwards  found  to  be 

1  The  name,  tornado,  was  originally  applied  some  two  centuries  or  more  ago,  to  the 
violent  thunder  squalls  that  frequent  the  western  equatorial  coast  of  Africa.  It  is  of  Spanish 
origin,  although  not  a  Spanish  word,  and  refers  to  the  rapid  shifting  of  the  wind  on  the  out- 
burst of  the  thunder  squall.  In  this  country,  however,  the  word  has  been  applied  for  about 
a  century  to  the  smaller  whirling  storms  with  pendant  funnels,  within  larger  storms.  To 
add  to  the  confusion  of  terms,  our  tornadoes  are  often  called  cyclones.  Newspaper  reports 
of  destructive  storms  seldom  call  them  by  the  proper  name,  and  in  many  instances  fail  to 
give  descriptions  by  which  the  true  character  of  the  storm  can  be  recognized.  In  this  book 
the  name,  cyclone,  will  be  used  only  for  large  areas  of  low  pressure,  so  well  defined  on  the 
weather  maps,  with  broad  sheets  of  clouds,  rain  or  snow  of  greater  or  less  amount,  and 
inflowing  spiral  winds,  seldom  of  destructive  strength  on  land,  and  occurring  with  us  even 
more  frequently  and  with  greater  strength  in  winter  than  in  summer;  while  toni;t<l<>, 
and  its  marine  synonym,  waterspout,  will  be  used  as  described  in  this  chapter. 


LOCAL    STORMS.  273 

sixteen  inches  in  diameter  and  sixty  feet  long,  was  seen  to  float  out  from  the 
black  vortex,  and  sail  around  to  all  appearances  as  light  as  a  feather. 

The  winds  in  the  tornado  vortex  attain  an  incredible  violence.  Houses  are 
torn  to  pieces,  and  their  fragments  are  scattered  for  hundreds  of  feet  along  the 
track  of  the  storm.  Trees  are  torn  up  by  the  roots  or  broken  from  their 
stumps  and  stripped  of  their  smaller  branches.  Men  are  carried  violently 
through  the  air,  falling  at  last  with  such  force  as  to  inflict  fatal  injuries  or 
cause  instant  death.  It  is  noteworthy  however  that  the  number  of  deaths 
reported  in  this  country  from  violent  winds  in  recent  years  is  less  than  the 
number  of  deaths  caused  by  lightning.  Cattle  have  been  impaled  by  flying 
boards.  Heavy  objects,  such  as  plows,  logs  or  chains,  are  carried  many  feet. 
Shingles,  clothing  and  papers  have  been  found  a  mile  or  more  from  where  the 
wind  caught  them  up.  Chickens  are  stripped  of  their  feathers.  Nails  are 
driven  into  boards.  The  variety  of  effects  is  endless ;  the  scene  of  destruction 
produced  by  the  passage  of  a  tornado  through  a  village  is  terrible  in  the 
extreme.  The  village  of  Grinnell,  Iowa,  was  thus  laid  waste  on  June  17, 
1882.  Eochester,  Minn.,  was  devastated  on  Aug.  21,  1883.  Many  other 
examples  of  tornadoes,  famous  for  their  violence,  could  be  added. 

It  is  manifest  from  these  accounts  that  whirling  tornadoes  and  outrushing 
thunder  squalls  should  not  be  confounded ;  although  it  is  probable  that  many 
of  the  latter  have  been  described  under  the  former  name.  They  are  alike  only 
in  their  associations  with  thunder  storms,  their  suddenness  and  their  brief 
duration.  Tornadoes  greatly  exceed  squalls  in  violence,  while  squalls  greatly 
exceed  tornadoes  in  the  breadth  of  country  over  which  they  are  felt  as  they 
advance.  Wtten  both  of  these  subordinate  but  violent  winds  occur  in  a  single 
thunder  storm,  the  squall  would  be  the  forerunner  of  the  tornado;  but  the 
tornado  would  be  felt  on  only  a  small  part  of  the  district  over  which  the  squall 
had  swept. 

267,  Regions  and  seasons  of  occurrence.  Tornadoes  are  more  frequent 
in  the  Mississippi  valley  and  in  certain  of  the  southern  states  than  in  other 
parts  of  this  country.  They  have  however  been  reported  in  all  the  states  east 
of  the  great  plains,  and  they  are  known  in  less  frequent  occurrence  in  Europe 
and  other  parts  of  the  world.  Being  distinctly  associated  with  thunder  storms, 
tornadoes  are  found  to  occur  in  greatest  number  in  the  warmer  months ;  but  in 
the  southern  states  they  are  sometimes  reported  during  the  warmer  spells  of 
the  colder  months.  They  are  more  frequent  in  the  warmer  afternoon  hours 
and  very  few  are  recorded  as  occurring  in  the  early  morning. 

Like  thunder  storms,  tornadoes  frequent  the  later  part  of  warm  spells  ; 
that  is,  the  western  part  of  areas  of  warm  southerly,  sirocco-like  winds,  or  the 
southerly  or  southwesterly  quadrant  of  cyclonic  storms.  It  is  by  means  of 
this  relation  of  tornadoes  to  larger  atmospheric  disturbances  that  they  may 


274  ELEMENTARY    METEOROLOGY. 

some  day  be  predicted  in  a  general  way,  for  nearly  all  the  tornadoes  that  have 
been  recorded  since  the  preparation  of  our  daily  weather-maps  have  been  found 
to  lie  south  or  southeast  of  a  cyclonic  center,  at  a  distance  varying  from  two 
to  eight  hundred  miles  from  it.  The  prognostics  of  tornadoes  are  therefore 
sultry,  moist,  southerly  winds  bearing  heavy  clouds,  often  portentous  in  form, 
agitation  and  coloring. . 

The  most  remarkable  example  of  the  relation  of  tornadoes  to  cyclones  yet 
noted  was  on  February  19,  1884,  when  some  forty  tornadoes  occurred  in  the 
southern  states  between  morning  and  midnight.  The  morning  weather-map 
of  that  day  (Fig.  64)  showed  a  trough-like  cyclonic  storm  of  considerable 
intensity  central  in  Illinois.  Its  moist  sirocco  indraft  was  drawn  from  the 
Gulf  over  the  southern  states  with  a  temperature  of  50°  or  80°  ;  while  the 
western  states  were  occupied  by  west  and  northwest  winds  with  a  temperature 
near  freezing,  and  in  the  far  northwest  even  10°  or  20°  below  zero.  All  of  the 
tornadoes  reported  on  this  disastrous  day  occurred  near  the  western  boundary 
of  the  area  occupied  by  the  sirocco.  During  the  morning,  they  were  reported 
in  Mississippi  and  western  Alabama.  In  the  afternoon,  the  cyclonic  center 
had  moved  over  southern  Michigan  ;  and  at  this  time  the  tornadoes  were  noted 
in  eastern  Alabama  and  Georgia.  By  evening,  the  cyclonic  center  had 
advanced  to  Lake  Huron  ;  tornadoes  were  then  formed  only  in  eastern  Georgia 
and  the  Carolinas.  Thus  for  the  entire  day,  the  district  in  which  the  violent 
whirlwinds  were  generated  stood  in  a  definite  relation  to  the  center  of  the 
,-*  larger  cyclonic  storm  and  its 


V 


N 

<•   > 

~"*    ' 


""£•    system    of    inflowing    winds. 
j  The  thunder  storms   of  this 

^  day    and    the    places    of    its 

^/  heavier  rains   have  not  been 

^%  j, 

specially  studied  ;  but  it  may 
,  7,j  /    ~  \v  be    expected  that  their  area 

"*>  —  marched  eastward  in  the  same 

*  /  ,  ,  manner  as  the  tornado  area. 

'       ?HMV*  n/r  •       -i 

'      \f»i*  Many    similar    examples 

might  be  given,  although  none 
have  the  number  of  tornadoes 
reported  on  the  disastrous  day 
described  above.  In  1893, 
March  25,  April  7  and  13 
^  possessed  warm  sirocco  winds 

"  in   which  numerous   and   de- 

structive local  storms  were  developed  in  the  Mississippi  valley. 

Fig.  101  represents  the  positions  of  13/5  tornadoes,  recorded  during  1884, 
with  respect  to  the  center  of  the  cyclones  in  whose  winds  the  smaller  whirls 


_/ 


IOOQ  M 


LOCAL    STORMS. 


275 


were  formed.  The  direction  of  movement  was  not  reported  for  those  indicated 
by  dots.  It  is  apparent  that  nearly  all  of  the  tornadoes  are  limited  to  a  definite 
space,  south-southeast  of  the  cyclonic  center. 

268.    Convectional  origin  of  tornadoes.     In  view  of  the  prevailing  associa- 
tion of  tornadoes  with  the  great  cumulus  mass  of  thunder  storms,  and  of  the 


FIG.  102. 

definite  relation  of  tornadoes  to  cyclones,  we  cannot  hesitate  to  refer  them  to 
some  special  form  of  convectional  action,  determined  not  alone  by  immediate 
and  local  sunshine  at  the  warmer  hours  of  the  day,  but  more  largely  by  the 
importation  of  air  masses  of  different  temperatures  and  humidities  from 
diverse  regions.  Thunder  storms  have  already  been  referred  for  the  most 
part  to  the  same  opportunity,  and  it  remains  to  be  seen  what  difference  of 
conditions  shall  determine  the  occurrence  of  thunder  storms  alone  and  of 
thunder  storms  with  tornado  funnels  beneath  them.  The  best  suggestion  yet 
offered  for  the  development  of  tornadoes  in  thunder  storms  is  based  on  the 
inferred  occurrence  of  exceptionally  strong  updrafts  here  and  there  in  the 
thunder  clouds.  A  side  view  of  a  distant  thunder  storm  often  indicates 
the  existence  of  locally  strong  updrafts  by  the  rise  of  certain  of  the  thunder 
heads  to  a  greater  height  than  the  rest ;  as  appears  in  the  accompanying 
sketch,  Fig.  102,  drawn  on  August  26,  1886,  looking  northward  from  Phila- 
delphia at  a  great  thunder  cloud  whose  distant  base  was  lost  in  the  hazy 
lower  air.  These  updrafts  are  thought  to  depend  in  turn  on  the  local  occur- 
rence of  an  unduly  warm  and  moist  mass  of  air,  which  may  consequently 
ascend  to  great  heights  in  the  atmosphere.  This  supposition  is  further 
indicated  by  the  prevailing  association  of  tornadoes  with  thunder  storms  of 
great  activity,  in  which  the  rain  is  heavier  than  usual,  and  from  which  hail 
not  infrequently  falls.  The  assistance  derived  from  the  liberation  of  latent 
heat  from  condensing  vapor  is  of  essential  importance  in  this  process ;  and  it 


276  ELEMENTARY   METEOROLOGY. 

must  be  remembered  that  the  occurrence  of  condensation  at  high  temperatures 
is  particularly  effective  in  retarding  the  cooling  of  ascending  currents,  and 
hence  in  aiding  their  ascent ;  because  the  decrease  of  vapor  capacity  is  then 
so  rapid. 

It  should  be  carefully  borne  in  mind  that  the  convection  here  considered 
does  not  depend  on  an  atmospheric  instability  that  is  determined  simply  by 
the  immediate  and  local  warming  of  the  lower  layers  of  air  by  sunshine,  even 
though  this  process  may  occasionally  produce  dust  whirlwinds  of  somewhat 
destructive  strength :  nor  does  the  instability  depend  on  the  long  quiet  brood- 
ing of  the  air  day  after  day  under  strong  sunshine,  such  as  that  which  gives 
rise  to  the  tropical  cyclones  in  the  doldrums.  The  instability  that  produces 
thunder  storms  and  tornadoes  is  believed  to  depend  on  the  importation  of 
unlike  masses  of  air  from  different  sources  into  close  neighborhood  and  into 
such  relative  positions  that  convectional  overturning  is  a  necessary  result. 
Some  meteorologists  have  questioned  the  sufficience  of  convection  as  a  cause 
for  the  excessive  violence  of  tornadoes ;  and  have  therefore  appealed  to  the 
action  of  electricity  or  of  some  even  more  mysterious  agency.  But  here,  as  in 
thunder  storms,  no  definite  connection  has  yet  been  shown  between  the  various 
suggested  agencies  and  the  actual  processes  of  the  storm  winds  ;  while  in  all 
cases,  the  action  of  convection  is  in  accord  both  with  the  conditions  in  which 
local  storms  occur  and  with  the  processes  that  they  exhibit. 

269  The  vortex  of  tornadoes.  The  development  of  a  whirling  motion  has 
already  been  shown  to  be  a  necessary  feature  in  the  growth  of  violent  cyclones  ; 
a  simple  radial  convectional  inflow  is  unable  alone  to  cause  winds  of  destructive 
strength.  It  is  the  same  with  tornadoes.  Their  destructive  winds  are  not 
radial  inflows ;  but  they  become  violent  only  when  a  vorticular  whirl  is 
developed. 

In  this  connection,  it  is  important  to  distinguish  between  bodily  rotation, 
such  as  that  of  a  wheel  or  of  the  earth,  and  vorticular  whirling,  as  in  a  water 
eddy  or  atmospheric  vortex.  In  the  former,  all  parts  rotate  at  a  constant 
angular  velocity  about  the  central  axis,  and  the  linear  velocity  increases  with 
the  distance  from  the  axis.  In  the  latter,  the  angular  and  the  linear  velocity 
rapidly  increase  towards  the  axis.  It  is  for  this  reason  that  the  tornado  vortex 
is  dangerous  only  at  a  small  distance  around  the  funnel  cloud. 

In  nearly  all  cases  where  the  direction  of  tornado  whirling  has  been 
determined  in  this  country,  either  by  observation  of  the  funnel  cloud  or  by  the 
distribution  of  objects  overturned  or  blown  about  by  the  whirling  winds,  it  lias 
been  found  to  be  from  right  to  left ;  that  is,  in  the  same  direction  as  that  of 
the  cyclonic  spirals  of  this  hemisphere.  It  is  possible  that  in  some  cases  the 
direction  of  turning  may  depend  on  the  accident  of  greater  strength  of  inflow 
on  one  side  than  on  the  other;  but  as  a  rule,  the  systematic  turning  from  right 


LOCAL    STORMS.  277 

to  left  is  manifestly  dependent  on  a  constant  cause,  such  as  the  earth's  rotation. 
It  cannot,  however,  be  supposed  that  the  area,  from  which  the  inflow  comes  at 
the  base  of  a  tornado,  is  large  enough  to  introduce  a  determining  deflective 
effect  from  the  earth's  rotation  ;  and  therefore  the  prevailing  direction  of  tornado 
whirls  should  be  regarded  as  determined  by  the  vorticular  movement  of  the 
cyclonic  winds  around  the  center  of  low  pressure  ;  these  having  been  previously 
determined  by  the  earth's  rotation.  It  is  a  general  mechanical  principle  that 
when  a  small  whirl  springs  up  in  a  larger  whirl,  the  two  must  turn  in  the 
same  direction.  It  is  for  this  reason  that  our  cyclones  turn  in  the  same 
direction  as  the  whirl  of  the  circumpolar  winds.  The  same  principle  explains 
the  agreement  in  the  direction  of  rotation  and  revolution  of  the  planets  around 
the  sun  and  of  the  moon  around  the  earth.  Indeed,  all  these  larger  and 
smaller  turnings,  from  the  greatest  to  the  least,  must  be  regarded  as  the 
continued  inheritance  from  the  original  impulse  by  which  the  rotation  of  all 
the  bodies  in  the  solar  system,  including  the  sun,  was  determined. 

270.  Ferrel's  theory  of  tornadoes  therefore  begins  with  the  occurrence  of 
an  especially  active  convectional  ascending  current  within  a  thunder  storm;  the 
possibility  of  such  currents  being  indicated  by  the  greater  activity  of  ascent  in 
some  thunder  heads  than  in  others  ;  while  the  actual  occurrence  of  active 
ascending  currents  in  tornadoes  is  indicated  by  the  vertical  component  of  the 
whirling  winds  which  lifts  heavy  objects  high  above  the  earth.  The  ascending 
current  draws  on  the  lower  warm  and  moist  air  for  its  supply ;  but  as  the  air 
makes  part  of  a  large  slowly-whirling  cyclonic  storm,  the  tornado  inflow  must 
develop  a  central  whirling  vortex,  which  shall  turn  in  the  same  direction  as  the 
parental  cyclone.  As  the  inflow  is  drawn  in  from  the  margin  towards  the  axis, 
slowly  ascending  at  the  same  time,  the  whirling  component  of  its  velocity  is 
greatly  increased,  and  when  near  the  center,  it  may  attain  an  irresistible 
violence.  At  a  moderate  distance  above  the  ground,  perhaps  a  few  hundred 
feet,  the  effect  of  friction  is  so  small  that  the  inflow  becomes  almost  a  perfectly 
circular  whirl  of  extremely  high  velocity  close  around  the  axis,  forming  a 
central  core  of  low  pressure,  very  similar  to  that  of  the  eye  of  a  tropical 
cyclone.  But  the  lower  air  is  prevented  by  friction  with  the  ground  from 
attaining  so  great  a  whirling  velocity  ;  its  centrifugal  force  is  therefore  less 
than  that  of  the  stronger  whirl  above  it ;  and  it  is  consequently  drawn  rapidly 
into  the  overhanging  core  of  low  pressure.  It  is  therefore  believed  that  the 
spiral  inrush  of  the  lower  air  into  the  low-pressure  core  made  by  the  higher 
whirl  constitutes  the  destructive  blast  of  the  tornado. 

The  student  should  guard  against  forming  too  rigid  a  conception  of  this 
theoretical  process.  The  activity  of  the  ascending  current  and  the  violence  of 
the  whirling  inflow  must  vary  from  time  to  time ;  the  axis  of  the  tornado  need 
not  be  vertical,  but  may  incline  somewhat  to  one  side  or  another  ;  it  need  not 


278  ELEMENTARY    METEOROLOGY. 

be  a  straight  line,  but  may  slowly  twist  and  writhe,  after  the  fashion  of  the 
empty  axial  core  of  water  eddies,  easily  observed ;  the  low  pressure  of  the  core 
must  vary  with  the  strength  of  the  lateral  inflows,  which  cannot  be  of  uniform 
value.  When  allowance  is  made  for  the  natural  irregularities  by  which  the 
ideal  process  is  complicated,  it  may  be  fairly  claimed  that  the  convectional 
theory  of  tornadoes  gives  a  reasonable  explanation  of  all  the  phenomena  that 
have  been  observed  in  these  storms. 

271.  Central  low  pressure  of  tornadoes.  The  inferred  low  pressure  of 
the  tornado  core  has  never  been  determined  by  observation;  and  probably 
never  can  be ;  but  analogy  with  other  whirls  renders  its  occurrence  highly 
probable.  The  circumpolar  whirl  of  the  terrestrial  winds  has  been  found 
competent  to  reverse  the  expected  high  pressure  of  the  cold  polar  regions  into 
low  pressure  ;  and  the  stronger  whirl  around  the  south  pole  causes  a  lower 
pressure  than  that  which  is  produced  by  the  slower  whirl  around  the  north 
pole ;  indeed,  if  it  were  not  for  the  resistances  caused  by  the  intermixture 
of  upper  and  lower  currents  in  cyclones  and  anticyclones,  the  polar  pressures 
might  be  much  lower  than  they  now  are.  Again,  the  central  low  pressure  of 
tropical  cyclones,  initiated  by  high  temperature,  is  greatly  intensified  by  the 
development  of  centrifugal  force  in  its  whirling  winds.  It  is  therefore 
reasonable  to  believe  that  the  excessively  rapid  whirl  of  the  winds  in  the 
tornado  vortex  close  around  the  axis  must  develop  a  vastly  greater  centrifugal 
force  than  occurs  even  in  tropical  cyclones  ;  and  the  occurrence  of  low  pressure 
within  such  a  vortex  cannot  be  doubted.  Various  facts  confirm  this  expec- 
tation. A  number  of  examples  have  been  reported  in  which  the  walls  of 
buildings  have  been  blown  outward,  as  if  by  the  explosive  expansion  of  the 
inside  air,  when  the  low  pressure  of  the  tornado  core  passed  overhead.  It  is 
possible  that  other  explanations  may  be  offered  for  this  peculiar  fact ;  but  none 
appear  so  reasonable  as  this  one.  For  example,  a  critical  observer,  describing 
the  destruction  of  a  factory  in  a  tornado  at  Arlington  (then  West  Cambridge), 
Mass.,  in  1852,  stated :  —  "  The  whole  effect  produced,  and  to  my  own  mind 
well  and  clearly  denned,  was  precisely  what  we  should  have  if  we  could 
suddenly  place  in  a  vacuum  a  building  filled  with  atmospheric  air  of  ordinary 
tension.  Even  the  foundation  walls  were  inclined  outwards,  and  there  was 

• — ^  every  evidence  of  a  force  acting 
from  the  interior  to  the  exterior." 
In  some  tornadoes,  it  is  reported 
that  corks  have  been  drawn  from 
empty  bottles,  as  if  by  the  ex- 
pansion of  the  air  from  within. 

The  atmospheric  pressure  ob- 
served along  an  east  and  west  line,  south  of  the  center  of  an  ordinary  cyclonic 


LOCAL    STORMS.  279 

storm,  may  be  indicated  by  a  concave  curve,  as  ABC,  Fig.  103.  If  a  thunder 
storm  occurs  about  the  time  of  lowest  pressure  —  that  is,  near  the  western 
border  of  the  southerly  winds  —  it  causes  a  slight  rise  of  the  barometer,  not 
from  increased  weight,  but  from  the  downward  reaction  of  the  rapidly 
expanding  and  ascending  mass  of  air  aloft ;  this  is  represented  by  the  modified 
curve,  ADEFC.  Now  if  a  tornado  occurs  in  such  a  thunder  storm,  its  whirling 
vortex  causes  an  extremely  low  pressure  within  a  slender  cylindrical  core  ;  and 
the  previous  curve  would  become  ADEGHJFC.  The  changes  of  pressure 
described  for  the  cyclonic  storm  and  for  the  thunder  storm  are  matters  of 
ordinary  observation ;  those  drawn  for  the  tornado  are  matters  of  reasonable 
inference. 

272.  The  tornado  funnel  cloud.  Accounts  of  tornadoes  and  water  spouts 
frequently  mention  the  descent  of  their  funnels  from  the  heavy  overhanging 
clouds,  as  if  there  were  actually  some  descending  motion ;  but  there  is  good 
reason  for  believing  that  the  descent  is  only  a  deceptive  appearance.  While  it 
is  generally  true  that  the  movement  of  clouds  indicates  the  movement  of  the 
air  in  which  they  are  formed,  this  is  not  always  the  case.  The  base  of  an 
ordinary  cumulus  cloud  is  fixed  at  a  certain  height,  although  the  air  is 
constantly  rising  through  it.  Stationary  clouds  stretching  out  from  mountain 
peaks,  or  standing  in  fixed  waves,  while  the  winds  in  which  the  clouds  are 
formed  are  moving  onward,  convince  us  that  the  outline  of  a  cloud  merely 
marks  the  limit  of  a  space  within  which  the  vapor  of  the  air  is  condensed  into 
visible  cloud  particles.  The  lower  front  edge  of  thunder  storm  clouds  may 
often  be  seen  growing  to  windward ;  the  eastward  advance  of  the  space  within 
which  the  ascending  air  is  cooled  by  expansion  being  more  rapid  than  the 
westward  ascent  of  the  inflowing  wind.  The  tornado  funnel  cloud  is  even 
a  more  striking  example  of  this  contradiction  between  apparent  and  real 
motion.  The  funnel  seems  to  descend,  because,  as  Franklin  clearly  said  in 
1753,  the  moisture  is  condensed  "  faster  in  a  right  line  downward  than  the 
vapors  [cloud  particles]  themselves  can  climb  in  a  spiral  line  upwards." 

This  may  be  explained  by  Fig.  104.  Suppose  the  observer  is  looking 
north  towards  the  funnel  EF.  Consider  now  the  condition  of  the  air  at  A,  at 
a  moderate  height  above  a  point,  B,  which  lies  several  hundred  feet  southwest 
of  the  vortex.  The  temperature  and  humidity  of  the  air  at  A  is  such  that 
if  it  ascends  vertically,  it  would  become  cloudy  at  the  height,  H,  in  the  base  of 
the  great  overhanging  thunder  cloud.  Instead  of  rising  vertically,  the  air 
from  A  moves  along  an  inflowing  and  gradually  ascending  spiral  path,  ADC, 
towards  the  lower  pressure  of  the  tornado  core.  On  reaching  the  point  C, 
it  is  cooled  both  by  expansion  due  to  ascent,  AC?,  and  by  expansion  into  the 
low  pressure  core  ;  and  if  at  C  the  cooling  due  to  expansion  into  the  low 
pressure  of  the  vortex  equals  the  cooling  which  would  be  produced  by  ascend- 


280 


ELEMENTARY    METEOROLOGY. 


ing  through   the   additional    height,    C'ff,    the   inflowing   whirling   air   will 
become  cloudy. 

The  combination  of  these  two  causes  of  cooling  gives  full  explanation  of 
the  form  of  the  funnel  cloud.  It  first  appears  as  a  somewhat  depressed 
portion  of  the  overhanging  cloud  mass ;  and  from  this  it  is  inferred  that  the 
whirl  of  the  tornado  begins  high  above  the  surface  of  the  earth  and  extends  its 
action  downward,  as  might  have  been  expected  from  the  probable  form  of  the 
vertical  temperature  gradient  of  strong  thunder  storms,  as  given  in  Fig.  99, 
where  the  flatter  part  of  the  line,  DG,  locates  the  place  of  greatest  instability 
at  a  considerable  height  above  the  ground.  As  the  ascent  and  inflow  of 


FIG.  104. 

the  growing  tornado  become  stronger,  its  whirl  is  strengthened,  and  the 
amount  of  cooling  due  to  expansion  into  the  axial  core  increases  ;  hence  the 
funnel  cloud  forms  at  lower  and  lower  levels.  Finally,  in  tornadoes  of  full 
strength,  the  funnel  seems  to  reach  down  to  the  ground,  and  in  the,  lan^ua^c 
of  ordinary  descriptions,  "it  destroys  everything  that  it  strikes."  This  should 
be  interpreted  to  mean  that  if  the  violence  of  the  vortex  is  sufficient  to  cause 
cloudy  condensation  close  down  to  the  ground,  it  must  also  be  strong  enough 
to  destroy  everything  in  its  path.  The  lower  part  of  the  funnel  is  of  less 
diameter  than  above ;  because  a  closer  approach  must  be  there  made  to  the 
hollow  core  before  cloudiness  begins.  Sometimes  the  lowest  part  of  the  funnel 
widens,  presumably  from  the  gathering  of  dusty  rubbish  from  the  ground. 

There  is  frequent  mention  made  in  the  descriptions  of  tornados  of  two 
clouds,  one  darker  than  the  other,  \vhirh  rush  together  and  form  the 
destructive  whirl.  Successive  observers  of  the  same  tornado  make  the  same 


LOCAL    STORMS.  281 

report ;  and  hence  it  must  be  supposed  that  this  process  is  a  continuous  one. 
In  such  cases,  it  is  not  the  rushing  together  of  the  clouds  that  causes  the 
destructive  wind,  but  the  rushing  in  and  whirling  around  of  the  wind  that 
continually  creates  the  clouds  and  carries  them  inwards  towards  the  vortex. 
After  lasting  half  an  hour  or  an  hour,  and  leaving  a  path  of  greater  or  less 
destruction  across  the  country,  the  tornado  weakens,  as  if  the  supply  of 
exceptionally  warm  and  moist  air  on  which  its  life  depends  were  then 
exhausted,  or  as  if  the  upward  path  of  escape  were  deformed  and  confused. 
The  funnel  gradually  withdraws  from  the  ground,  disappearing  in  the  clouds 
above,  and  the  storm  is  over.  But  when  one  tornado  is  formed,  others  often 
appear  in  the  same  neighborhood,  and  thus  a  succession  of  whirls  may  traverse 
the  country,  as  in  the  example  of  February  19,  1884  ;  although  the  number 
of  tornadoes  then  reported  is  altogether  exceptional. 

273.  The  progression  of  tornadoes.     Tornadoes  in  this  country  generally 
move  easterly  or   northeasterly,  sometimes   southeasterly ;    seldom   in   other 
directions.     Their  velocity  of  progression  commonly  varies    from   twenty  to 
forty  miles  an  hour.     As  the  vortex  is  small  and  its  progression  rapid,  less 
than  a  minute  suffices  to  carry  it  past  any  given  point.      The    duration  of 
tornadoes  ranges  from  half  an  hour  to  an  hour  or  more ;  hence  their  length  of 
path  may  reach  thirty  to  fifty  or  more  miles.     Examples  in  which  a  much 
greater  length  of  path  is  reported  probably  consist  in  reality  of  two  or  more 
whirls,  forming  successively  almost  in  the   same   line.     Although   occurring 
within  the   area   of   southerly  surface  winds,    the   advance   of   tornadoes   is 
more  accordant  with  the  direction  of  the  higher  currents ;  hence  it  may  be 
inferred  that  the  place  of  escape  of  the  ascending  currents  is  born  along  within 
the  great  nimbus  clouds  by  the  westerly  overflowing  winds. 

Instances  were  mentioned  in  Section  257  of  local  thunder  storms  that  stood 
distinctly  within  the  area  of  southerly  winds  and  that  were  followed  as  well  as 
preceded  by  high  temperatures.  It  is  not  yet  clear  from  reported  records 
whether  this  is  generally  or  occasionally  the  case  with  tornadoes  also.  It  is 
frequently  stated  that  the  warm,  sultry  weather  which  preceded  a  tornado  was 
followed  by  cooler  weather  ;  but  it  is  not  yet  made  certain  that  tornadoes 
always  occur  close  along  the  belt  of  separation  between  the  southerly  and 
westerly  winds.  Special  observation  might  well  be  directed  to  this  question. 

274.  Protection  from  tornadoes.     The  approach  of  tornadoes  is  so  rapid 
and  their  arrival  follows  so  soon  after  their  first  appearance  that  there  is 
very  seldom  time  for  those  who  happen  to  be  on  their  path  to  escape  their 
fury.     If  a  tornado  is  seen  approaching  obliquely,  so  that  the  continuation  of 
its  path  will  carry  it  to  one  side  of  the  observer,  but  not  far  distant,  he  should 
lose  no  time  in  running  away  from  it ;   and  if  a  distance  of  five  hundred  feet 


282  ELEMENTARY   METEOROLOGY. 

is  gained  before  its  arrival,  serious  danger  is  pretty  surely  avoided.  If  the 
tornado  seems  to  be  coming  directly  towards  the  observer,  it  is  safer  to  rim  to 
the  northern  side  of  its  path  ;  for  on  that  side  its  whirling  winds,  blowing 
to  the  west,  are  weakened  by  the  progressive  velocity  of  the  whirl  which 
carries  it  to  the  east.  For  this  reason,  the  path  of  the  vortex  does  not  lie  along 
the  middle  of  the  path  of  destructive  action,  but  somewhat  north  of  the  middle. 

In  case  of  the  sudden  approach  of  a  tornado  unseen,  as  at  night,  when  its 
coming  is  only  known  by  the  roaring  of  its  winds,  the  southwest  corner  of  a 
house  cellar  is  regarded  as  the  safest  place  of  refuge.  Sometimes  special 
underground  cellars  are  prepared  beforehand  for  case  of  need ;  and  these  are 
provided  with  a  bar,  an  axe  and  a  saw  by  the  more  cautious,  in  order  to  assure 
means  of  escape  in  case  the  house  is  demolished  overhead. 

Although  individual  tornadoes  are  excessively  destructive  to  everything 
that  lies  in  their  path,  yet  their  limits  of  action  are  so  narrow  and  our  country 
is  so  wide  that  the  danger  from  tornadoes  at  any  one  place  is  much  less  than 
has  been  supposed.  The  annual  loss  to  the  country  by  fire  and  flood  greatly 
exceeds  that  caused  by  tornadoes. 

275.  Observation  of  tornadoes.  The  rapid  passage  of  a  roaring  tornado 
is  not  a  time  when  deliberate  observation  of  its  action  can  be  expected.  Yet 
if  a  person  happens  to  stand  a  few  hundred  yards  on  one  side  of  its  path,  a 
close  scrutiny  of  the  funnel  and  the  clouds  overhead  might  be  safely  made. 
If  such  a  person  had  in  mind  the  interpretation  of  tornado  action  as  here 
presented  essentially  in  accordance  with  Ferrel's  theory,  his  attention  might 
be  critically  directed  to  such  features  of  the  storm  as  need  examination  in 
repeated  occurrence.  He  should  examine  the  funnel  to  determine  the  character 
of  its  rotation ;  he  should  look  closely  to  discover  the  movements  of  cloud 
wisps  or  of  trees  and  other  objects  in  the  funnel ;  he  should  examine  the  rela- 
tion of  movements  in  the  funnel  to  those  in  the  greater  overhanging  clouds  j 
the  behavior  of  the  two  clouds,  whose  rushing  together  is  so  often  mentioned  ; 
the  occurrence  of  descending  clouds  sometimes  noted  at  some  distance  to  one 
side  of  the  vortex.  In  a  larger  way,  the  relation  of  the  tornado  to  the  fall  of 
rain  or  hail  from  the  thunder  cloud  should  be  examined,  for  heavy  precipitation 
commonly  occurs  at  a  moderate  distance  from  the  tornado,  rather  than  imme- 
diately around  its  vortex ;  the  distribution  of  temperatures  and  the  relation  of 
electric  action  to  the  funnel  also  need  attention,  as  different  accounts  vary 
greatly  in  these  matters. 

After  the  passage  of  the  tornado,  the  peculiar  effects  of  its  destructive 
winds  should  be  recorded :  a  map  should  be  prepared  to  show  the  place  of 
buildings  destroyed  and  of  trees  overturned,  and  the  path  of  objects  carried 
by  the  winds.  That  tornadoes  are  destructive  is  only  -too  well  known,  but  the 
particular  method  of  their  action  as  indicated  in  their  effects  should  be  care- 


LOCAL    STORMS.  283 

fully  determined.  As  a  rule,  the  evidence  of  vorticular  action  in  the  attitudes 
of  overturned  trees  and  in  the  distribution  of  fragments  of  buildings  is  not  so 
clear  as  might  be  expected  from  the  visible  whirling  of  the  winds  in  the  funnel 
cloud.  Greater  or  less  confusion  results  in  the  first  place  from  the  increased 
strength  of  the  indraft  on  the  southerly  side  of  the  whirl,  as  already  explained  ; 
and  in  the  second  place  because  the  destruction  of  objects  is  not  accomplished 
all  at  once,  but  irregularly  and  successively,  according  to  their  resistance  and 
to  the  storm's  strength.  For  example,  on  the  northern  or  left  side  of  the 
track,  trees  are  sometimes  blown  down  almost  in  a  forward  direction,  as  if 
by  the  rear  inflow  ;  and  therefore  the  backward  whirl  of  the  winds  on  this 
side  is  more  or  less  confused  with  forward  action.  Yet  sometimes  a  field  of 
grain  may  record  a  partial  circuit  of  the  winds,  as  if  they  had  suddenly  fallen 
on  it,  and  prostrated  all  the  stalks  at  once  in  a  sweeping  curve,  with  a  radius 
of  several  hundred  feet.  The  indications  of  whirling  motion  found  in 
prostrated  trees  is  generally  as  follows :  trees  blown  down  backward  are 
found  only  on  the  northern  side  of  the  track  of  the  vortex ;  they  are  often 
crossed  over  by  other  trees  falling  to  the  southeast  or  east,  as  if  by  the  later 
winds  in  the  rear  of  the  whirl.  On  the  south  of  the  vortex,  many  trees  are 
laid  nearly  parallel  with  the  track ;  but  those  first  blown  down  turn  more  to 
the  north,  and  those  last  overthrown  turn  more  to  the  south.  Sometimes  the 
distribution  of  identifiable  fragments  from  buildings  gives  indication  of  a 
curved  course  through  the  air :  at  the  Lawrence,  Mass.,  tornado  of  July,  1890, 
the  southern  windows  of  a  house  south  of  the  track  were  broken  by  rubbish 
carried  from  other  houses  several  hundred  feet  to  the  northwest.  Such  facts 
as  these  should  be  written  down  on  the  ground,  in  order  that  no  mistakes  of 
memory  may  occur. 

276.  Waterspouts.  The  behavior  of  waterspouts  at  sea  is  so  closely  like 
that  of  the  spouts  caused  by  tornadoes  when  they  cross  rivers  or  ponds  that 
there  can  be  no  doubt  of  the  essential  similarity  of  these  vorticular  storms  on 
land  and  sea.  Waterspouts  possess  a  tapering  funnel  cloud,  first  seen  as  a 
small  pendant  from  the  under  surface  of  the  overhanging  clouds ;  then  appar- 
ently descending  to  sea  level,  where  the  greatly  agitated  waters  rise  to  meet 
it.  Although  these  spouts  seem  to  draw  water  up  from  the  sea,  they  consist 
of  fresh  water  for  the  greater  part ;  and  hence  must  be  regarded  as  the  product 
of  vapor  condensed  from  the  air.  There  is  an  old  account  of  a  vessel  on 
which  a  waterspout  fell.  A  flood  of  water  poured  on  the  master,  so  that  he 
was  obliged  to  lay  hold  of  what  was  nearest  to  him  to  escape  being  washed 
overboard.  He  was  asked  afterwards  if  he  had  tasted  the  water.  "  Taste  it," 
said  he,  "  I  could  not  help  tasting  it ;  it  ran  into  my  mouth,  nose,  eyes  and 
ears  ! "  "  Was  it,  then,  fresh  or  salt  ?  "  "  As  fresh,"  said  the  captain,  "  as 
ever  I  tasted  spring  water  in  my  life." 


284  ELEMENTARY    METEOROLOGY. 

Waterspouts  seem  to  be  most  common  in  the  warmer  and  calmer  seas  ;  but 
they  are  also  recorded  in  middle  or  higher  temperate  latitudes  and  in  the 
presence  of  moderate  winds  ;  and  a  good  number  of  examples  have  been 
recorded  on  the  Gulf  Stream  in  winter,  in  the  presence  of  cold  westerly  winds 
blowing  off  the  colder  land.  In  some  of  these  cases,  the  instability  on  which 
the  spouts  depend  may  arise  from  local  causes  ;  but  in  others,  and  particularly 
in  the  last  mentioned,  the  instability  appears  to  depend  on  the  importation  of 
air  with  a  temperature  much  lower  than  that  of  the  water  over  which  it 
advances,  much  as  was  the  case  with  tornadoes  on  land. 

A  few  records  have  been  made  of  the  appearance  of  descending  currents 
within  the  core  of  a  waterspout ;  from  which  it  must  be  concluded  that  the 
interior  and  exterior  parts  of  the  spout  have  opposite  motions.  It  has  been 
suggested  that  these  descending  central  currents  are  streams  of  rain,  falling 
from  above  into  the  nearly  empty  core  within  the  whirling  spout ;  and  that 
such  currents  are  more  likely  to  characterize  waterspouts,  where  the  lower 
inflow  close  to  the  sea  surface  is  little  retarded  by  friction,  and  hence  cannot 
enter  the  core  easily;  while  in  tornadoes  on  land,  descending  currents  would 
seem  to  be  less  likely  to  occur,  because  their  place  is  taken  by  inflow  and 
ascent  of  the  surface  currents,  whose  centrifugal  force  is  not  sufficient  to  hold 
them  out  of  the  low  pressure  close  to  the  axis.  The  reported  descending 
currents  in  waterspouts  may  therefore  be  compared  with  the  inferred  descend- 
ing currents  in  the  eye  of  tropical  cyclones  at  sea ;  while  the  presence  of  only 
ascending  currents  in  tornadoes  may  be  compared  with  the  inferred  ascending 
currents  about  the  usually  cloudy  center  of  cyclonic  storms  in  our  latitudes 
and  on  land.  Closer  observation,  with  suggestions  of  this  kind  in  mind,  may 
some  day  determine  these  minor  points. 


THE   CAUSES   AND   DISTRIBUTION    OF   RAINFALL.  285 

CHAPTER    XII. 

THE   CAUSES   AND   DISTRIBUTION    OF   RAINFALL. 

277.  Causes  of  rainfall.  When  vapor  is  condensed  in  sufficient  quantity, 
it  falls  from  the  clouds  and  reaches  the  earth  as  rain  or  snow.  All  forms  of 
atmospheric  precipitation  are  included  under  the  general  term,  rainfall. 

The  occurrence  of  rain  or  of  its  winter  equivalent,  snow,  is  in  nearly  all 
cases  associated  with  overgrown  clouds ;  and  in  the  temperate  zone  at  least, 
these  are  usually  products  of  cyclonic  or  of  local  storms.  It  occasionally  happens 
that  rain  or  snow  falls  from  the  clear  sky ;  but  this  is  highly  exceptional,  and 
its  amount  is  small.  The  processes  which  produce  clouds  may  always,  if 
carried  far  enough,  bring  forth  rainfall.  Mention  has  already  been  made 
of  this  in  the  case  of  the  great  overgrown  cumulus  clouds  of  warm  summer 
weather.  The  small  clouds  of  morning  are  succeeded  by  greater  ones  towards 
noon,  and  these  by  even  more  massive  clouds  two  or  three  hours  later.  Such 
cloud  masses  may  be  many  miles  long  and  wide,  and  at  least  four  or  six  miles 
high,  drifting  along  in  the  high-level  currents  of  the  atmosphere  and  yielding 
heavy  rain  to  the  earth  below.  Cyclonic  cloud  masses  are  many  times  larger. 

The  cause  of  rainfall  is  not  far  to  seek.  Every  cloud  particle  serves 
as  a  center  for  additional  condensation  in  the  cooling  saturated  air.  The 
particles  must  be  of  slightly  unequal  size ;  the  larger  ones  are  not  so  easily 
borne  up  in  the  air  as  the  smaller  ones  are;  collisions  must  occur,  and  when 
two  drops  coalesce,  the  resulting  larger  drop  tends  to  fall  more  rapidly  than 
either  drop  fell  before.  In  the  vertical  or  obliquely  ascending  currents,  in 
which  clouds  and  rain  are  so  commonly  formed,  the  drops  may  grow  to  a  size 
large  enough  to  cause  them  to  fall  through  the  rising  air.  As  the  temperature 
of  the  drops  is  then  lower  than  that  of  the  damp  air  through  which  they  fall, 
continuous  condensation  is  provoked  on  their  cool  surfaces.  Collisions  will  be 
more  frequent  than  before,  and  the  drops  will  grow  more  and  more  rapidly  as 
they  fall  to  the  base  of  the  cloud,  where  their  largest  size  is  reached  :  3V  to 
^  of  an  inch  in  fine  rain ;  ^  of  an  inch  or  more  in  the  heavy  pattering  ruins 
of  summer.  On  falling  below  the  cloud  into  non-saturated  air,  the  size  of  the 
drops  decreases  by  evaporation.  In  dry  regions  or  seasons,  it  is  not  uncommon 
to  see  a  trail  of  rain  falling  from  the  base  of  a  lofty  rain  cloud,  and  entirely 
disappearing  before  reaching  the  earth.  Such  rain  clouds  may  be  seen  rising 
over  the  ridges  of  our  Rocky  Mountains,  bringing  dark  streams  of  rain  along 
beneath  them ;  and  yet  only  a  few  large  drops  reach  the  thirsty  ground  ;  but 
when  the  lower  air  is  damp,  as  under  winter  cloud  sheets,  the  rain  that 
falls  from  the  cloud  for  the  most  part  reaches  the  earth.  Sometimes  the 


286  ELEMENTARY    METEOROLOGY. 

rain  drops  of  winter  storms  are  frozen  into  clear  ice  pellets,  when  falling  from 
a  warm  upper  current  into  cold  surface  air;  this  should  be  called  frozen  rain, 
and  not  hail,  which  is  of  quite  different  form  and  associations,  as  is  explained 
below. 

278.  Snow.     In  case  the  process  of  condensation  occurs  at  temperatures 
below  32°,  the  vapor  then  crystallizes  as  it  condenses  and  forms  snow  flakes  or 
ice  needles  of  varied  forms,  but  always  presenting  angles  of  60°  and  120°, 
characteristic  of   crystallized  water.      In   quiet  snow  falls,  the  crystals  are 
remarkably  perfect  and  by  far  the  greater  number  of  them  are  of  one  form. 
As  each  crystal  is  developed  in  this  case  from  a  single  center,  their  growth 
must  be  explained  by  continuous  condensation  on  some  initial  nucleus,  and 
not  by  collision,  such  as  presumably  occurs  in  the  formation  of  faster  falling 
rain  drops,  or  in  the  matted  flakes  of  windy  snow  storms.     In  milder  winter 
weather,  when  snow  falls  into  a  warmer  surface  layer  of  air,  it  partly  melts  and 
reaches  the  ground  as  sleet.     A  more  complete  melting  would  deliver  it  as 
rain ;  and  it  is  probable  that  most  of  our  winter  rain  has  had  the  form  of  snow 
while  it  was  still  in  the  higher  clouds.     Indeed,  a  part  of  our  summer  rains 
also  may  be  formed  as  snow  in  the  upper  parts  of  the  thunder  storm  clouds  ; 
and  if  caught  on  high  mountains,  the  snowy  form  is  preserved.     When  precipi- 
tation occurs  in  the  polar  regions  at  temperatures  lower  than  — 5°  or  — 10°, 
small  ice  needles  and  not  snow  flakes  are  formed. 

279.  Hail  consists  of  compacted  ice  and  snow,  often  arranged  in  roughly 
concentric  layers,  taking  the  form  of  little  pellets  or  balls,  commonly  called 
hail-stones.     It  does  not  fall  in  winter,  but  is  a  common  accompaniment  of 
thunder  storms,  even  though  they  occur  chiefly  in  the  warmer  regions  and 
seasons  of  the  globe.     Occasionally  hail-stones  show  a  crystalline  structure. 
They  are  sometimes  of  remarkable  size,  up  to  several  inches  in  diameter  ;  and 
they  may  then  cause  serious  destruction  to  trees,  crops  and  buildings.     Hail 
should  be  distinguished  from  the  clear  icy  pellets  of  frozen  rain,  which  some- 
times fall  in  the  winter  season,  but  never  in  the  summer ;  and  also  from  round 
pellets  of  snow  of  loose  structure,  called  soft  hail,  again  a  product  of  winter 
instead  of  summer. 

The  association  of  hail  with  active  convectional  storms  in  the  warm  season 
suggests  that  it  is  produced  by  the  freezing  of  rain  drops  that  have  been  formed 
at  low  levels,  and  that  are  then  carried  upwards  by  the  central  ascending 
currents  to  altitudes  where  the  t.cinpnrature  is  very  low;  there  they  become 
coated  with  a  layer  of  snmv.  increasing  in  size  until  they  fall  through  the  less 
active  currents  near  the  margin  of  the  storm;  still  increasing  in  size  by  further 
condensation  on  their  cold  surfaces  during  descent  through  the  clouds  ; 
and  perhaps  again  carried  inward  at  the  base  of  the  cloud  and  upward  once 


THE    CAUSES    AND    DISTRIBUTION    OF    RAINFALL.  287 

more  through  its  center;  untiFat  last  they  become  too  heavy  for  further 
carriage  and  fall  to  the  ground.  Before  their  fall,  a  curious  rattling  may 
sometimes  be  heard  in  the  air,  as  if  caused  by  their  noisy  collisions.  As  a 
general  rule,  hail  is  larger  and  more  plentiful  in  violent  than  in  moderate 
thunder  storms. 

Although  associated  with  electrical  storms,  there  is  no  sufficient  reason 
for  regarding  electricity  as  the  chief  agent  in  the  production  of  hail.  It  is  true 
that  hail-stones  may  still  be  so  distinctly  electrified  on  reaching  the  ground 
that  they  may  leap  again  a  few  inches  into  the  air  ;  not  only  by  elastic  rebound, 
but  as  if  by  electric  repulsion.  Yet  as  with  the  other  products  of  thunder 
storms,  the  electric  condition  of  hail-stones  seems  to  be  essentially  a  result  of 
the  processes  of  their  formation,  and  not  primarily  a  cause  of  their  formation. 
The  supposition  that  they  rise  and  fall  by  electric  attraction  and  repulsion 
between  the  two  cloud  layers  of  which  thunder  storms  have  been  said  to  consist 
does  not  find  support  in  the  present  knowledge  of  the  structure  of  thunder 
storms,  or  in  the  estimates  of  the  force  that  electrical  attraction  and  repulsion 
could  attain  in  the  atmosphere. 

Like  the  rain  of  thunder  storms,  their  hailfall  is  distributed  in  belts, 
whose  breadth  depends  on  the  size  of  the  storm,  and  whose  length  depends 
on  its  duration  and  velocity  of  progression.  In  the  larger  linear  thunder 
storms,  hail  seems  to  fall  only  from  certain  parts  where  the  storm  is  of 
great  violence. 

280.  Conditions  of  rainfall.  There  can  be  no  question  that  ascensional 
movements  in  the  atmosphere  of  whatever  cause  are  the  most  effective  means 
of  condensing  vapor  «o  plentifully  and  rapidly  as  to  produce  rain  :  hence  the 
greater  rainfall  of  regions  frequented  by  cyclonic  storms  or  thunder  storms, 
where  the  air  is  given  a  vertical  component  in  its  movement;  hence  also 
the  greater  rainfall 'of  mountains,  especially  on  their  windward  slopes,  where 
the  air  is  forced  to  ascend  in  crossing  them.  There  are,  however,  two  other 
processes  of  cooling  already  mentioned  in  connection  with  the  development  of 
cyclonic  and  other  clouds,  that  should  be  referred  to  here  again.  The  first  of 
these  is  the  movement  of  masses  of  air  poleward  so  that  their  attitude  with 
respect  to  sunshine  is  changed,  and  radiation  coming  to  be  in  excess  causes 
their  temperature  to  fall.  These  currents  may  become  cloudy  in  so  great  a 
mass  as  to  yield  rain :  hence  our  southerly  cyclonic  winds  or  moist  siroccos 
are  not  only  generally  cloudy  but  frequently  rainy  as  well.  In  this  country, 
it  is  as  a  rule  only  cyclonic  winds  that  move  poleward  directly  and  rapidly 
enough  to  produce  speedy  condensation  in  this  way. 

A  second  process  deserving  mention  is  that  which  appears  when  a  current 
of  moist  air  from  over  the  ocean  advances  upon  a  cold  winter  land.  Its  mass 
may  then  become  cooled  not  simply  by  conduction,  which  extends  but  a  little 


288  ELEMENTARY  METEOROLOGY. 

way  above  the  earth,  but  by  radiation  from  the  air,  especially  from  the  cloudy 
air,  to  the  ground.  Landward  winds  may  thus  become  cloudy  in  large  volume, 
even  to  the  point  of  yielding  rain ;  but  when  rain  appears,  the  winds  are 
generally  in  this  case  also  found  to  be  within  the  influence  of  a  cyclonic 
storm,  in  which,  as  has  just  been  stated,  some  vertical  motion  aids  the  other 
causes  of  cooling.  Rainfall  as  a  result  of  the  mixture  of  two  masses  of 
saturated  air  at  different  temperatures  does  not  seem  to  be  common. 

The  exceptional  occurrence  of  rain  falling  from  a  clear  sky  is  called  serein. 
It  is  rarely  noted  and  is  not  well  understood. 

281.  Cooling  caused  by  rainfall.     Distinction  must  be  carefully  made 
between  the  adiabatic  increase  of  temperature  by  compression  in  descending 
currents   of   air,  and  the   maintenance  of  a  nearly  constant  temperature  in 
falling  rain  drops  or  snow  flakes,  which  cannot  be  compressed.     While  the 
descent  of  a  current  of  air  from  the  height  of  a  thunder  storm  summit  would 
give  it  a  high  temperature  at  sea  level,  the  descent  of  rain  drops  or  snow 
flakes  from  similar  heights  causes  a  distinct  cooling  of  the  warmer  lower  air 
through  which  they  fall.     A  considerable  part  of  the  lowering  of  temperature 
that  is  commonly  noted  during  a  summer  rainfall  must  be  ascribed  to  this 
process. 

282.  Variation  of  rainfall  with  altitude.     Clouds  attain  their  greatest 
frequency  at  a  moderate  height  above  the  earth,  averaging  perhaps  a  half  mile 
or  a  mile,  because  the  dew-point  to  which  the  air  .is  cooled  by  the  various 
processes  of  cloud-making  is  most  commonly  encountered  at  this  altitude; 
their  greatest  density  is  found  at  about  the  same  height  because  condensation 
at  greater  altitudes  and  hence  at  lower  temperatures  is  attended  by  less 
plentiful  exclusion  of  vapor.     For  the  same  reason,  the  greatest  measure  of 
rainfall  in  a  given  region  is  not  at  sea  level  or  at  the  level  of  the  earth's 
surface,  but  at  a  certain  moderate  altitude  in  the  atmosphere,  varying  witli 
the  region  and  the  season.     At  lower  levels,  some  of  the  precipitation  falls 
from  the  clouds  into  non-saturated  air  and  is  redissolved  ;  at  greater  altitudes, 
less  precipitation  is  formed.     In  the  Alps,  the  maximum  is  found  at  about 
.'JOOO  or  4000  feet  in  winter ;  but  in  summer,  when  the  air  is  relatively  drier, 
the  maximum  appears  to  be  at  a  greater  height  than  the  records  reach.     In 
the  southern  ranges  of  the  Himalaya,  the  level  of  maximum  summer  rainfall 
is  at  4000  feet;  and  it  is  near  this  height  that  Cherrapunji  lies  on  the  Khasiu 
hills,  where  the  heaviest  rainfall  of  the  world  is  found  (Sect.  301):  in  winter 
the  maximum  rainfall  is  at  about  20,000  feet.     In  this  country,  an  increase 
of   rainfall   up   to  a  certain  height  undoubtedly  prevails,  especially  in  tin- 
western  territory,  where  the  air  is  generally  so  dry;    but  records  are  not, 
yet  made  to  determine  it.      The  rainfall  maps  of  our  western  area,  based 


THE   CAUSES   AND   DISTRIBUTION   OF   RAINFALL.  289 

on  observations  made  for  the  most  part  on  the  plains  and  lowlands,  probably 
do  not  represent  the  full  value  of  the  precipitation  of  that  mountainous 
region.  The  lofty  upland  surface  of  the  Aquarius  plateau  in  southern 
Utah  is  well  watered  and  bears  an  extended  forest,  while  the  lower  plateau 
country  about  it  is  a  desert;  but  no  sufficient  indication  of  this  contrast  can 
at  present  be  given  on  the  charts,  on  account  of  the  absence  of  high-level 
records. 

In  Europe,  where  records  of  rainfall  have  been  more  generally  maintained 
on  highlands  and  lowlands,  the  rainfall  charts  correspond  to  a  remarkable 
degree  with  the  relief  of  the  country.  The  highlands  of  the  northern  Atlantic 
coast  are  shaded  dark  for  a  heavy  rainfall ;  the  Pyrenees  possess  a  plentiful 
rainfall  between  the  dry  lowlands  of  southern  France  and  the  semi-arid  plateau 
of  northern  Spain.  The  Alps  form  a  center  of  strong  precipitation.  The 
small  rainfalls  of  the  basins  of  Bohemia  and  Hungary  are  surrounded  by 
heavier  rainfalls  on  the  enclosing  mountains  of  Germany  and  Austria.  The 
mountains  of  the  Caucasus  have  a  heavy  rainfall  between  the  dry  steppes  of 
southern  Russia  and  the  arid  plateaus  of  Asia  Minor.  The  more  accurate  the 
charts,  the  closer  this  relation  appears  to  be. 

The  rainfall  on  mountain  ranges  is  greater  on  their  windward  slopes. 
Little  difference  in  this  respect  is  perceptible  on  ranges  so  low  as  our  Appa- 
lachians, and  on  which  the  rainy  winds  blow  from  different  sides.  The  Sierra 
Nevada  has  more  rainfall  on  its  long  western  slope  than  on  its  precipitous 
eastern  descent.  The  Pyrenees  are  watered  chiefly  on  their  northern  side; 
the  Himalaya  on  their  southern.  The  equatorial  Andes  have  heavy  rains  on 
their  eastern  slopes ;  the  Chilean  Andes  receive  rain  chiefly  on  the  western 
slope. 

283.  Measurement  of  rainfall.  It  is  desired  to  measure  the  depth  of  the 
sheet  of  water  that  would  lie  on  level  ground  after  a  rain  if  none  of  the  water 
were  lost  by  evaporation  or  by  soaking  into  the  soil.  This  is  done  by  exposing 
a  cylindrical  vessel  or  rain  gauge  to  the  storm  and  measuring  the  depth  of  rain 
or  snow  that  it  receives.  A  good  gauge  should  have  a  truly  circular  rim ; 
a  diameter  of  at  least  five  or  six  inches  being  recommended.  The  edge 
should  be  .sharp,  with  a  vertical  face  on  the  inside.  The  gauge  should  be 
placed  in  a  level  and  open  field,  removed  if  possible  from  all  trees  and  build- 
ings by  at  least  twice  their  height ;  it  should  be  fastened  in  position,  to  avoid 
overturning  by  the  wind.  The  rim  should  stand  a  foot  above  the  ground ;  it 
should  be  carefully  levelled.  Once  placed  in  a  well-selected  situation,  subse- 
quent change  should  be  carefully  avoided  ;  but  if  required,  a  full  account  of 
the  change  should  be  entered  in  the  record  book.  In  order  to  avoid  loss  by 
evaporation,  a  movable  funnel  is  generally  placed  within  the  gauge,  thus 
protecting  the  water  that  lies  beneath  it  from  loss  to  the  air. 


290  ELEMENTARY   METEOROLOGY. 

The  measurement  of  the  amount  of  rain  collected  is  best  done  by  pouring 
the  water  from  the  gauge  into  a  measuring  tube  of  a  certain  smaller  diameter, 
so  that  its  area  shall  be  one  tenth  of  that  of  the  gauge.  The  water  then  rises 
in  the  tube  to  ten  times  the  true  depth  of  the  rainfall.  This  magnified  depth 
is  then  measured  by  a  graduated  stick,  the  record  being  made  to  a  hundredth 
of  an  inch.  Record  should  be  made,  if  possible,  at  the  close  of  every  storm 
and  always  once  a  day ;  although  some  observers  measure  the  rainfall  only  at 
a  certain  hour  every  day,  without  regard  to  the  time  when  the  rainfall  ceased. 
The  amount  measured  should  always  be  entered  in  the  record  book  before  the 
measuring  tube  is  emptied. 

The  easy  drifting  of  light  snow  makes  its  measurement  a  matter  of  much 
uncertainty.  It  can  seldom  be  correctly  determined  by  the  amount  that  is 
caught  in  gauges,  unless  the  wind  has  been  very  light.  It  is  recommended 
that  observers  measure  the  amount  of  snow  lying  on  the  ground  in  open  woods, 
where  drifting  is  slight.  The  measure  may  be  made  with  a  stick  while  the 
snow  is  on  the  ground ;  or  a  section  of  snow  may  be  cut  by  the  rim  of  the 
gauge,  and  the  amount  of  snow  thus  secured  may  be  melted  and  measured  as 
rain.  Melting  is  best  done  by  adding  a  measured  amount  of  warm  water. 
Light  snow  is  generally  eight  or  ten  times  as  deep  as  the  corresponding 
amount  of  rain. 

Records  of  rain  and  snow  from  gauges  on  buildings  in  cities  are,  as  a  rule, 
defective,  because  of  the  eddies  of  wind  by  which  too  much  or  too  little  rain 
is  carried  into  the  gauge.  Such  measures  may  serve  to  indicate  generally 
whether  the  fall  is  light  or  heavy,  as  is  required  in  weather  reports ;  but  they 
should  not  be  accepted  for  the  climatic  tables  of  a  district.  Self-registering 
rain  gauges  have  been  devised,  by  which  the  fall  is  recorded  every  five 
minutes,  but  these  are  seldom  employed.  No  satisfactory  means  have  been 
devised  to  measure  the  rainfall  at  sea :  only  'the  time  of  occurrence,  the 
relative  frequency  and  the  estimated  amount  of  rainfall  are  reported  in  marine 
observations. 

284.  Records  of  rainfall.  The  data  desired  in  this  connection  are :  the 
amount  of  precipitation  in  every  separate  fall,  but  when  brief  showers  follow 
one  another,  the  whole  fall  may  be  measured  at  once ;  the  time  of  beginning 
and  ending ;  and  if  possible,  the  direction  of  the  wind  at  these  times.  The 
rainfall  of  every  day  should  be  determined  late  in  the  evening  during  a 
long  storm.  When  the  last  day  of  a  month  is  rainy,  the  measurement  should 
be  delayed  till  as  near  midnight  as  possible,  in  order  to  give  a  correct  monthly 
total.  The  records  of  rainfall  are  generally  summarized  as  follows :  Total 
rainfall  for  each  month;  snow  on  the  ground  on  the  15th  and  at  end  of  month  ; 
share  of  monthly  fall  in  the  form  of  snow ;  dates  of  first  and  last  snowfall ;' 
number  of  rainy  days ;  that  is,  of  days  on  which  more  than  one  hundredtli  of 


THE   CAUSES   AND   DISTRIBUTION   OF    EAINFALL.  291 

an  inch  of  rain  or  snow  fell ;  maximum  daily  fall  of  the  year.  After  the 
records  have  been  carried  on  over  a  period  of  years,  the  normal  or  mean 
monthly  and  annual  rainfalls  should  be  determined.  In  our  country,  at 
L -;ust  ten  years  record  is  needed  before  the  mean  annual  total  can  be 
<l«'tcrmined  with  acceptable  accuracy;  and  a  thirty  years  record  is  needed 
for  the  monthly  means,  as  the  fluctuation  in  their  values  is  often  great.  The 
maximum  fall  for  a  given  month  may  be  many  fold  greater  than  the  minimum. 
The  plus  or  minus  departure  of  the  fall  of  each  month  from  its  normal  should 
be  stated.  The  average  number  of  days  in  each  month  on  which  a  hundredth 
or  a  tenth  of  an  inch  falls,  or  the  probability  of  rainy  days,  constitutes  an 
important  climatic  factor.  It  is  desirable  to  determine  the  average  number  of 
rainy  spells  for  each  month  and  the  average  fall  in  each  spell.  The  share 
of  rainfall  supplied  by  winds  or  storms  of  different  character  should  receive 
attention ;  and  the  summary  for  the  year  should  subdivide  the  total  as  far 
as  possible  according  to  origin. 

Xo  regions  where  continuous  records  have  been  maintained  are  found  to  be 
absolutely  rainless.  Southeastern  California  and  western  Arizona  have  certain 
stations  where  the  mean  annual  rainfall  is  under  two  inches,  and  where  in 
single  years  less  than  an  inch  has  been  gauged.  Other  desert  regions,  such  as 
central  Arabia  or  the  interior  of  the  Sahara  may  have  even  less,  but  records 
have  not  been  kept  there  to  make  this  certain.  The  greatest  annual  rainfall  is 
found  in  India  and  the  East  Indies,  where  many  stations  record  over  a  hundred 
inches  a  year.  The  most  remarkable  of  these  is  Cherrapunji,  at  an  elevation 
of  4,455  feet  on  the  southern  slope  of  a  subordinate  range  of  the  Himalaya 
mountains,  north  of  the  head  of  the  Bay  of  Bengal,  with  an  average  annual 
rainfall  of  474  inches,  of  which  over  400  fall  in  the  five  months  from  May 
to  September,  or  during  the  summer  monsoon.  A  fall  of  40.8  has  been 
measured  at  this  station  in  a  single  day  (June  14,  1876),  and  over  600  inches 
or  fifty  feet  have  been  collected  in  certain  years.  The  average  fall  for  the  five 
rainy  months  is  almost  three  inches  a  day.  Here  truly  "it  never  rains  but  it 
pours."  The  rainfall  of  Mahableshwar  on  the  bold  western  slope  of  southern 
India  at  an  altitude  of  4,5^0  feet  is  hardly  less  remarkable,  reaching  an  average 
of  261  inches,  of  which  251  fall  from  June  to  September  inclusive.  It  is 
noteworthy  that  at  a  little  distance  east  of  this  station  on  the  relatively  even 
plateau  of  the  Deccan,  the  rainfall  is  reduced  to  less  than  20  inches  a  year. 

The  greatest  mean  annual  rainfall  of  this  country  is  101.87  at  Neah  Bay, 
Wash,  (or  including  Alaska,  111.72  at  Sitka),  and  the  greatest  single  annual 
fall  is  123.23  at  Neah  Bay  in  1886  (or  140.26  at  Sitka  in  1886).  The  follow- 
ing items  have  a  statistical  interest  in  this  connection  : l  Excessive  monthly 
rainfalls :  at  Upper  Mattole,  California,  January,  1888,  41.63 ;  at  Alexandria, 

1  See  Greeley's  American  Weather,  in  which  many  facts  of  this  kind  are  collected. 


292  ELEMENTARY   METEOROLOGY. 

Louisiana,  June,  1886,  36.9.  Excessive  daily  falls  amounting  to  ten  or  twelve 
inches  have  been  recorded  at  several  stations.  Brief  downpours  :  at  Washing- 
ton, D.  C.,  June  27,  1881,  2.34  in  37  minutes  ;  at  Philadelphia,  July  26,  1887, 
0.62  in  seven  minutes.  Much  heavier  falls  have  undoubtedly  occurred  in 
cloud  bursts,  such  as  happen  in  the  western  states  and  territories,  but  no 
precise  measure  of  their  amount  is  at  hand.  The  amount  of  rainfall  in  single 
storms  is  sometimes  excessive  over  a  large  area  of  country,  producing  disastrous 
floods.  On  February  11-13,  1886,  more  than  five  inches  were  recorded  over 
an  area  of  5,000  square  miles  in  southeastern  New  England;  the  Johnstown 
flood  in  Pennyslvania,  May  30-June  1,  1889,  was  estimated  at  over  eight 
inches  upon  an  area  of  about  12,000  square  miles,  with  somewhat  less  fall  on  a 
much  larger  surrounding  area,  causing  a  terrible  destruction  of  life  and  property. 
In  northern  India,  September  17-18,  1880,  ten  inches  of  rain  fell  over  an  area 
of  10,000  square  miles  ;  its  weight  being  7,248,000,000  tons.  It  is  manifest 
that  the  liberation  of  latent  heat  from  so  vast  an  amount  of  rain  —  or  the 
release  of  so  great  a  supply  of  stored  solar  energy  —  must  be  the  means  of 
doing  an  enormous  amount  of  work. 

285.  Relation  of  rainfall  and  agriculture.  When  the  annual  rainfall  is 
under  eighteen  inches,  agriculture  can  seldom  be  safely  practised  without 
irrigation.  Grazing  then  becomes  the  chief  occupation,  as  is  now  the  case 
over  a  large  extent  of  our  western  plains,  between  the  98°  meridian  and  the 
Rocky  Mountains ;  and  the  people  return  in  a  measure  to  the  roving  life 
characteristic  of  the  aboriginal  inhabitants  of  semi-arid  regions.  When  the 
rainfall  is  less  than  twelve  inches  a  year,  the  region  is  reduced  to  a  desert,  and 
the  water  supply  is  too  small  to  be  of  service  in  irrigation,  unless  in  small  areas, 
or  on  the  banks  of  large  rivers.  On  the  other  hand,  the  tropical  regions  where 
the  rainfall  rises  above  a  hundred  inches  a  year  are  so  luxuriantly  overgrown 
as  to  make  their  occupation  a  difficult  matter.  The  general  rainfall  of  the 
eastern  part  of  our  country  or  of  western  Europe,  with  an  annual  total  varying 
from  forty  to  eighty  inches  equably  distributed  through  the  year,  is  an  amount 
under  which  human  occupations  are  best  developed. 

The  distribution  of  rainfall  through  the  year  is  a  matter  of  great  moment. 
The  northeastern  part  of  the  United  States  is  favored  in  having  the  average 
value  of  the  precipitation  in  successive  months  comparatively  equable ; 
droughts  are  exceptional,  but  when  occurring  are  found  in  one  season  about  ;is 
frequently  as  another.  In  Florida,  the  summers  are  wet  and  the  winters  are 
comparatively  dry.  In  eastern  Nebraska,  there  is  a  similar  distribution  of 
rainfall  from  dry  winters  to  wet  summers.  In  California,  the  reverse  is  true  ; 
the  summers  have  a  continuous  drought,  making  the  ground  dry  and  dusty  ; 
the  winters  are  cloudy  and  wet.  These  variations  will  be  found  to  depend 
chiefly  on  the  system  of  the  general  winds. 


THE   CAUSES   AND   DISTRIBUTION   OF   RAINFALL.  293 

The  irregularity  in  the  monthly  rainfall  is  an  important  matter,  especially 
in  regions  where  the  annual  supply  is  moderate.  Even  in  the  well-watered 
states  east  of  the  Mississippi,  a  deficiency  in  the  summer  rainfall  sometimes 
causes  droughts  ;  for  example,  the  southern  Atlantic  states  had  in  September, 
1888,  1.84  inches ;  while  the  same  month  of  the  preceding  year  had  9.47,  or 
over  five  times  as  much  :  this  variation  appears  to  result  chiefly  from  the 
variation  in  number,  paths,  and  activity  of  cyclones  in  different  years.  On 
the  margin  of  the  western  Plains,  where  the  total  rainfall  is  hardly  enough  for 
agriculture,  the  departures  from  the  normal  monthly  fall  are  of  more  serious 
import :  a  succession  of  rainy  seasons  tempts  settlers  further  and  further  west, 
and  when  a  series  of  drier  years  follows,  the  distress  occasioned  by  the  failure 
of  crops  becomes  a  calamity. 

In  India,  where  the  year  is  divided  into  three  seasons,  the  cold,  the  hot, 
and  the  wet  seasons,  the  crops  are  preserved  in  the  dry  season  by  irrigating 
canals  fed  from  rivers  rising  in  the  mountains,  or  in  a  smaller  way  by  water 
pumped  up  from  the  rivers  ;  if  the  water  in  the  rivers  in  insufficient,  or  if  the 
rains  arrive  late  or  are  deficient,  famine  results,  and  the  people  of  that  great 
country  die  by  the  thousands.  In  more  recent  years,  with  the  improvement 
of  the  irrigating  canals,  and  with  the  better  means  of  transportation  of  the 
plenty  of  one  province  to  supply  the  need  of  another,  the  danger  from  this 
source  is  lessened. 

286,  Snowfall.  In  regions  where  the  winter  snowfall  covers  the  ground 
to  a  thickness  of  a  foot  or  more  and  remains  unmelted  for  a  considerable 
period,  it  exercises  an  important  influence  on  the  temperature  of  the  air. 
Being  of  relatively  loose  texture,  it  is  a  poor  conductor  and  thus  very 
effectively  prevents  the  escape  of  heat  from  the  ground ;  at  the  same  time,  the 
surface  of  the  snow,  losing  its  own  heat  by  radiation  and  absorbing  very  little 
insolation,  falls  to  a  low  temperature,  and  thus  cools  the  air  lying  upon  it. 
The  presence  of  a  heavy  snow-cover  during  winter  thus  protects  the  ground 
from  deep  freezing  and  at  the  same  time  determines  the  occurrence  of  very  low 
temperatures  in  the  air. 

The  opening  of  spring  is  much  delayed  in  regions  where  the  winter  snows 
accumulate  to  a  considerable  depth ;  for  until  all  the  snow  is  melted,  the 
surface  cannot  gain  a  temperature  above  32°.  The  low  temperatures  of 
the  polar  regions,  where  ice  and  snow  prevail  and  last  through  the  brief 
solstitial  season  with  its  high  values  of  insolation,  have  already  been  explained 
in  this  way  (Sect.  88).  Near  our  eastern  coast,  where  rain  and  snow  rapidly 
succeed  one  another  in  winter  time,  it  frequently  happens  that  a  heavy  snow- 
fall will  be  almost  entirely  melted  a  few  days  later  by  a  mild  rain,  and  thus 
the  precipitation  of  two  storms  will  be  delivered  quickly  to  the  streams.  Our 
floods  of  winter  and  spring  are  chiefly  caused  in  this  way. 


294  ELEMENTARY   METEOROLOGY. 

Lofty  mountains  in  all  latitudes  and  plateaus  in  the  polar  regions  often 
receive  a  greater  supply  of  snow  in  the  cold  season  than  is  melted  in  the 
following  milder  season.  The  thickness  of  the  snow  cover  then  continually 
increases  until  a  movement  towards  lower  ground  is  established  to  dispose  of 
the  excessive  supply ;  if  lying  on  steep  mountain  slopes,  the  snow  falls  in 
avalanches  into  the  adjacent  ravines,  filling  them  to  a  great  depth.  With 
increasing  pressure  and  especially  with  the  aid  of  percolating  water  from 
surface  melting  and  from  occasional  rain  storms,  the  accumulated  snow  is 
gradually  welded  into  ice.  The  mass  thus  formed  slowly  creeps  downward, 
following  the  slope  of  the  ground,  and  enters  lower  and  milder  levels  as  a 
glacier ;  finally  ending  when  the  melting  of  its  extremity  balances  the  supply 
from  downward  creeping ;  or  in  the  polar  regions,  ending  in  the  sea  where  its 
margin  breaks  off  and  forms  icebergs,  which  are  then  borne  away  by  currents. 

The  value  of  snow  as  a  store  of  winter  precipitation  for  summer  use  is  very 
great  in  arid  regions ;  and  in  the  coming  century  we  may  expect  to  see  the 
water  of  spring  freshets  that  now  runs  to  waste  from  our  western  mountains 
utilized  in  large  part  by  detaining  it  in  reservoirs  in  the  upper  narrow  valleys, 
and  leading  it  out  along  the  valley  sides  when  desired  in  artificial  canals  until 
it  reaches  the  flat  divides  of  the  interstream  surfaces  on  the  arid  plains,  where 
it  can  then  be  distributed  over  large  areas.  This  is  already  done  in  a  small 
way,  but  such  works  will  have  to  be  greatly  increased  as  the  need  for  them 
becomes  more  and  more  pressing. 

287.  Ice  storms.     Regions  of  strongly  variable  temperature  are  subject  to 
occasional  winter  storms  in  which  the  precipitation  occurs  as  rain,  but  freezes 
as  soon  as  it  touches  any  solid  body,  such  as  the  branches  of  trees,  or  telegraph 
wires,  or  the  ground.     This  happens  when  the  ground  and  the  lower  air  have 
been  made  excessively  cold  during  a  spell  of  clear  anticyclonic  weather,  when 
a  moist  upper  current  in  advance  of  an  approaching  cyclone  brings  clouds  and 
rain  (Sect.  245).     Serious  damaere  is  caused  by  breaking  down  over- weighted 
wires  and  branches  at  such  times      Wires  may  be  increased  in  weight  ten  or 
twenty  fold ;  and  twigs  even  more  than  a  hundred  fold.     New  England  is 
particularly  subject  to  such  storms,  although  they  are  not  by  any  means 
common ;   in  the  winter  of  1886,  three  ice  storms  occurred  in  January  and 
February,  but  this  was  exceptional.     They  were  all  accompanied  by  northeast 
winds,  with  surface  temperatures  at  or  a  little  above  freezing,  while  similar  or 
slightly  higher  temperatures  prevailed  on  Mt.  Washington.    When  the  deposit 
of  ice  is  in  small  amount,  sufficing  only  to  glaze  the  surface  on  which  it  is 
formed,  it  is  called  a  "  silver  thaw." 

288.  Relation  of  rainfall  and  forests.     The  latter  sections  of  this  chapter 
will  make  it  clear  that  the  contrast  between  forested  and  barren  regions  depends 


THE    CAUSES    AND   DISTRIBUTION    OF    RAINFALL.  295 

on  the  general  winds  and  the  form  of  the  land  areas,  all  of  which  are  permanent 
physical  features  of  the  earth,  as  far  as  human  history  extends.  There  is, 
however,  a  very  general  impression  that  the  presence  of  forests  increases  the 
rainfall ;  and  that  the  destruction  of  forests  may  cause  the  rainfall  to  diminish 
so  much  as  to  reduce  fertile  regions  to  arid  sterility.  It  is  not  to  be  doubted 
that  the  clearing  of  forests  causes  great  fluctuations  in  the  volume  of  streams, 
especially  in  hilly  or  mountainous  regions.  The  streams  overflow  at  times 
of  heavy  rain,  when  the  undelayed  surface  water  rushes  down  the  slopes, 
washing  the  soil  along  with  it,  flooding  and  clogging  the  valleys  with  water 
and  sand,  and  thus  devastating  both  high  and  lowland ;  the  streams  almost 
disappear  in  droughts  when  the  unsheltered  ground  is  dried  and  springs 
weaken  their  flow;  but  it  is  quite  another  matter  to  affirm  that  the  amount 
of  rainfall  is  altered  by  the  destruction  of  forests.  There  are  few  actual 
measurements  that  can  be  appealed  to,  and  these  do  not  give  definite  answer 
to  the  problem ;  the  evidence  that  can  be  obtained  does  not  clearly  support 
the  popular  belief. 

Popular  opinion  is  also  disposed  to  believe  in  an  increase  in  the  rainfall  of 
our  semi-arid  western  Plains  by  means  of  tree  planting  and  agriculture  ;  but 
no  evidence  in  the  form  of  actual  records  has  been  adduced  to  prove  this  very 
hazardous  conclusion.  The  often-quoted  account  of  a  wholesale  planting  of 
trees  in  Egypt  in  the  early  part  of  this  century  and  a  consequent  increase 
of  rainfall  is  untrue ;  no  such  artificial  climatic  change  has  been  produced  in 
that  country,  and  none  need  be  expected  in  this. 

289.  Artificial  rain.  It  is  claimed  by  some  that  extensive  conflagrations 
promote  rainfall  by  exciting  a  convectional  overturning  in  the  atmosphere, 
which  then  grows  to  a  rain  storm;  and  by  others,  that  violent  concussions, 
such  as  the  firing  of  heavy  artillery  on  battlefields,  causes  rainfall  even  in 
times  of  drought.  The  advocates  of  these  theories  have  at  times  tried  to 
provoke  rain  artificially.  While  it  might  perhaps  be  possible  to  hasten  the 
overturning  of  an  almost  unstable  atmosphere  by  a  vast  conflagration,  it  would 
certainly  be  very  expensive  to  attempt  to  alleviate  the  unfavorable  conditions 
of  a  persistent  drought  or  the  long  dry  season  of  an  arid  region  by  this 
method,  and  we  need  not  expect  to  witness  its  successful  application.  As  for 
'the  efforts  to  produce  rain  by  firing  dynamite  and  other  explosives  in  Texas  in 
1891,  the  official  account  of  these  experiments  gives  every  indication  that 
only  a  few  pattering  drops  of  rain  were  caused  by  the  explosions,  and  these 
only  when  heavy  clouds  were  floating  overhead;  the  rains  that  followed  the 
longest  series  of  explosions  were  to  all  appearances  ordinary  summer  thunder 
storms  whose  path  happened  to  carry  them  over  the  places  where  the 
explosions  were  fired. 


296  ELEMENTARY    METEOROLOGY. 

290.  Rainbows.    When  falling  rain  is  illuminated  by  the  direct  rays  of  the 
sun,  a  rainbow  or  arc  of  prismatic  colors  is  seen  on  the  rain,  with  its  center 
opposite  the  sun,  and  a  radius  of  40-42^° ;  red  is  on  the  outside  of  the  arc,  and 
blue  011  the  inside.     An  additional  or  secondary  bow  is  sometimes  formed 
outside  of  the  first,  and  of  fainter  colors ;  its  radius  is  50-54°,  and  its  colors 
are  inverted  from  their  order  in  the  primary  bow.     Rainbows  are  produced  by 
a  complicated  process  of  refraction  of  sunlight  as  it  enters  and  passes  out  of 
the  rain  drops,  internal  reflection  of  the  light  within  the  drops,  and  interference 
of  the  rays  after  leaving  the  drops.    The  proper  understanding  of  the  problem 
requires  a  careful  study  of  optics. 

Rainbows  cannot  be  seen  from  lowlands  when  the  sun  is  high  above  the 
horizon ;  their  arc  increases  in  length  as  the  sun  comes  nearer  the  horizon ; 
and  at  sunrise  or  sunset  they  may  form  a  full  semicircle.  Observers  on 
mountain  summits  often  see  a  rainbow  of  more  than  a  semicircle  when  rain  is 
falling  through  the  air  below  them.  In  our  latitudes,  rainbows  are  most 
common  in  summer  afternoons  on  the  rear  of  receding  thunder  showers ; 
because  it  is  under  these  conditions  that  strong  and  nearly  horizontal  rays 
from  the  sun  fall  on  a  heavy  curtain  of  rain.  Another  hour  of  occurrence 
might  be  more  common  near  the  equator,  where  thunder  storms  commonly 
move  to  the  west ;  if  occurring  about  sunset,  the  bow  would  precede  the  rain 
instead  of  following  it.  Cyclonic  rains  seldom  produce  rainbows ;  their  rain 
area  is  generally  followed  by  so  large  an  area  of  cloud  before  the  clear  sky 
appears  that  the  sun  and  the  rain  are  seldom  visible  at  the  same  time  near  the 
opposite  horizon  points  ;  these  larger  rains  have  not  the  habit,  so  pronounced 
with  our  thunder  storms,  of  moving  away  to  the  eastward  about  the  time  of 
sunset,  and  leaving  a  brilliantly  clear  sky  immediately  behind  them. 

291.  Correlation  of  rainfall  with  the  circulation  of  the  atmosphere.     It 
appears  from  the  preceding  explanations  that  rainfall  is  in  practically  all  cases 
produced  by  some  movement  of  the  winds.     We  shall  therefore  now  consider 
the   distribution  .of   rainfall  over  the  world   in  quantity  and   in  season  of 
occurrence,  in  connection  with  the  general  and  more  local  circulation  of  the 
atmosphere.     We  gain  from  this  comparison  a  fuller  appreciation  of  both 
processes   than  if  either  is  considered  alone ;    at  the  same  time,  it  will  be 
seen  that  the  theory  of  the  general  winds,  presented  in  Chapter  VI,  receives 
additional  support  from  the  explanation  that  it  gives  of  the  distribution  of 
wet  and  dry  regions  and  seasons.     The  rational  correlations  of  the  facts  of 
temperature,  pressure,  winds  and  rainfall  constitute  the  strength  of  the  science. 

292.  Equatorial  rains.     The  calms  of  the  doldrums  have  already  been 
described  as  occupying  the  belt  of  low  pressure  around  the  equator.     They  are 
supplied  from  either  side  by  the  inflowing  trade  winds  which  loiter  here  on 


THE   CAUSES    AND    DISTRIBUTION    OF    RAINFALL.  297 

the  weakest  gradients,  allowing  the  air  to  reach  a  high  temperature  and 
humidity  and  to  expand  upward,  causing  an  overflow  aloft,  and  thus  establish- 
ing the  upper  currents  that  run  obliquely  towards  the  poles.  In  the  process 
of  expansion,  there  is  also  a  diurnal  convectional  movement,  caused  on  the 
^[iiatorial  lands  by  the  warming  of  the  lower  air,  but  on  the  oceans  presum- 
ably due  in  greater  part  to  the  upward  expansion  and  diffusion  of  the  plentiful 
vapor  there  taken  from  the  water  surface.  In  this  belt  of  warm  damp  air, 
the  noonday  witnesses  the  production  of  clouds,  followed  in  the  afternoon  or 
evening  by  the  occurrence  of  lively  showers  of  rain,  which  frequently  reach 
the  activity  of  violent  thunder  storms  ;  late  in  the  night  the  clouds  dissolve 
away,  and  in  the  morning  the  sky  is  generally  clear.  The  belt  of  doldrums  is 
therefore  also  known  as  the  equatorial  cloud  belt  and  as  the  belt  of  equatorial 
rains,  standing  in  strong  contrast  with  the  comparatively  dry  trade  wind  belts 
on  either  side.  The  equatorial  rainfall  is  estimated  at  about  100  inches.  Its 
large  amount  is  due  not  only  to  the  activity  of  the  convectional  processes  on 
which  it  depends,  but  also  and  largely  to  the  rapid  decrease  of  the  capacity 
for  vapor  when  air  cools  at  the  high  temperatures  prevailing  around  the 
equator. 

293.  Trade  wind  rains.  The  trade  wind  belts  over  the  oceans,  although 
of  a  rather  high  relative  humidity,  have  a  comparatively  light  rainfall  because 
the  temperature  of  their  winds  rises  as  they  flow,  and  their  capacity  for  vapor 
correspondingly  increases.  The  evaporation  that  they  cause  from  the  ocean 
surface  is  so  strong  that  a  slightly  greater  degree  of  salinity  is  recognized  in 
the  ocean  waters  within  their  limits,  the  trade  wind  belts  being  separated  by  a 
belt  of  less  saline  water  under  the  heavy  fall  of  the  equatorial  rains.  This  is 
illustrated  in  Fig.  105,  where  the  surface  water  of  the  trade  wind  areas  has 
densities  of  more  than  1.0270 ;  while  the  equatorial  belt  is  below  1.0265  or 
1.0260. 

If  the  trade  wind  encounters  a  mountainous  island  or  a  bold  continental 
coast,  the  driven  ascent  of  the  air  over  such  obstructions  requires  it  to  cool 
by  expansion,  thus  producing  clouds  and  generally  rain  as  well ;  precipitation 
of  this  kind  is  known  as  tropical  rainfall.  For  this  reason,  the  windward 
slopes  of  lofty  tropical  islands,  like  those  of  the  Antilles,  and  the  windward 
coasts  of  torrid  lands,  like  Guiana  and  southeast  Brazil,  are  well  watered, 
receiving  a  rainfall  of  from  sixty  to  a  hundred  or  more  inches  annually  ;  while 
the  leeward  slopes  are  comparatively  dry,  as  in  Peru  and  northern  Chile  on 
the  leeward  slope  of  the  Andes.  The  two  sides  of  the  Hawaiian  islands  are 
similarly  contrasted,  one  being  well  clothed  with  tropical  vegetation,  and  the 
other  being  comparatively  dry  and  barren.  In  central  America,  the  contrast 
between  the  well-watered  and  heavily-forested  eastern  slopes  and  the  drier  and 
more  open  western  slopes  has  in  great  part  determined  the  abandonment  of 


298 


ELEMENTARY    METEOROLOGY. 


the   former  to  the  aboriginal  tribes,  while  the  latter  have  become  the  seat 
of  European  settlement. 

An  occasional  cause  of  heavy  rainfall  in  certain  parts  of  the  trade  wind 
belts  is  found  in  the  furious  cyclones  that  traverse  them  on  curved  paths  from 
the  doldrums  to  the  temperate  zone.  Although  of  not  frequent  occurrence, 
these  cyclonic  rains  are  truly  torrential,  as  has  been  stated  in  Section  218. 


Fie.  105. 

Data  are  not  at  hand  to  define  closely  the  amount  or  distribution  of  rainfall 
from  this  cause  ;  but  on  the  Caribbean  sea  and  in  other  cyclonic-  regions  on 
the  ocean,  a  considerable  share  of  the  total  may  be  thus  produced.  Some 
of  the  Lesser  Antilles  have  their  greatest  rainfall  in  the  autumnal  cyclone 
season. 

294.  Trade  wind  deserts.  Wh«-n  tin-  trade  winds  blow  over  a  land  of 
moderate  elevation,  they  generally  reduce  its  surface  to  a  desert  by  depriving 
it  of  moisture  ;  for  it  is  not  the  quality  of  the  desert  rock  or  soil  that  makes 


THE   CAUSES   AND   DISTRIBUTION    OF   RAINFALL.  299 

it  barren,  but  simply  the  aridity.  Plant  life  is  almost  or  entirely  driven 
away ;  the  unsheltered  dust  produced  by  rock  disintegration  is  carried  oft' 
by  the  wind,  and  only  sand,  stones  and  rocky  ledges  remain.  Thus  the  great 
Sahara,  crossed  by  warming  and  drying  winds  from  southern  Europe  and  the 
Mediterranean  towards  the  equator,  has  extremely  little  rainfall  and  is  left  a 
barren  waste ;  excepting  in  the  more  lofty  mountainous  parts  of  its  surface, 
where  the  ascending  wind  becomes  rainy.  Arabia,  Persia,  and  a  large  part  of 
Australia  are  sterile  for  similar  reasons. 

The  greater  area  of  deserts  in  the  eastern  than  in  the  western  hemisphere 
is  the  result  of  the  outline  of  the  lands  and  the  trend  of  the  great  mountain 
chains.  In  the  eastern  hemisphere,  the  greatest  breadth  of  Africa  lies  under 
the  northeast  trade  winds  ;  it  is  a  desert  plateau  of  moderate  height  with  few 
mountains ;  a  similar  desert  surface,  but  more  broken  by  mountains,  is  con- 
tinued within  the  trade  wind  belt  across  Arabia  and  Persia  into  northwest 
India.  In  the  western  hemisphere,  the  corresponding  area  north  of  the 
equator  is  largely  oceanic,  the  American  continent  being  narrowest  in  the 
latitudes  of  the  northeast  trades,  and  widest  under  the  equatorial  rains. 
Moreover,  Mexico  and  Central  America  possess  mountains  and  table  lands  of  a 
considerable  altitude,  lying  directly  across  the  course  of  the  winds,  and  thus 
calling  forth  a  plentiful  rainfall  on  the  windward  slopes  at  least. 

South  of  the  equator,  Africa  has  a  good  supply  of  rainfall  on  its  moun 
ainous  coast  to  the  southeast,  where  the  moist  southeast  trade  from  the  Indiai, 
ocean  strikes  the  land ;  but  it  contains  a  large  area  of  moderate  rainfall  in  the 
interior,  and  towards  the  western  coast  the  desert  of  Kalahari  repeats  the 
aridity  of  the  Sahara,  but  on  a  smaller  scale.  Australia  presents  a  similar 
arrangement,  having  a  narrow,  well-watered  coastal  strip  on  the  southeast, 
while  the  interior  is  for  the  most  part  too  dry  for  occupation.  A  correspond- 
ing succession  of  parts  may  be  seen  in  torrid  South  America,  but  with  certain 
differences.  South  of  the  equator  and  towards  the  Atlantic  coast,  there  are 
plentiful  rains  on  the  mountain  slopes ;  further  inland  the  country  becomes 
lower  and  much  drier ;  it  is  almost  a  desert  in  the  trade  wind  latitudes  near 
the  eastern  base  of  the  Andes,  but  this  great  barrier  again  provokes  rainfall 
and  leaves  only  a  narrow  desert  strip  along  the  Pacific  coast. 

295.  The  horse  latitudes  or  belts  of  tropical  high  pressure  have  been 
explained  as  regions  of  gently  descending  air,  whence  the  trades  and  the 
surface  members  of  the  prevailing  westerlies  move  away  obliquely  on  either 
side.  As  ascending  air  cools  and  becomes  cloudy  and  rainy,  so  descending  air 
warms  and  becomes  dry  and  clear.  The  horse  latitudes  are  therefore  regions 
of  fresh  clear  air,  drier  than  the  trades  and  with  little  rainfall  in  their  light 
and  baffling  breezes.  The  contrast  between  the  equatorial  and  tropical  belts 
of  light  winds  and  frequent  calms  is  therefore  highly  instructive  when  con- 


300  ELEMENTARY    METEOROLOGY. 

sidered  in  connection  with  the  general  circulation  of  the  atmosphere ;  one 
being  sultry,  damp,  cloudy  and  rainy ;  the  other  fresh,  clear  and  relatively 
dry ;  just  as  the  theory  of  Chapter  VI  would  require.  The  growth  of  con- 
vectional  clouds  may  produce  local  rains  within  this  belt,  but  they  are  neither 
so  plentiful  nor  so  frequent  as  the  heavy  daily  rains  of  the  equatorial  belt. 

296.  The  stormy  rainfall  of  the  westerly  winds.  The  westerly  winds 
follow  on  the  poleward  side  of  the  horse  latitudes.  These  seldom  produce 
rain  from  their  own  action,  but  they  are  subject  in  both  hemispheres  to 
frequent  stormy  or  cyclonic  overturn  ings,  and  in  these  overturnings  the  vapors 
gathered  by  the  winds  from  the  oceans  are  condensed  to  cloud  sheets  and 
yield  plentiful  rain.  In  certain  parts  of  this  belt,  the  precipitation  is  greater 
and  more  frequent  in  amount  in  winter  than  in  summer,  because  in  winter  the 
activity  of  the  winds  is  greater  and  the  violence  of  the  storms  is  then 
increased ;  this  is  especially  apparent  on  the  oceans  and  on  western  coasts  in 
middle  and  higher  latitudes.  In  other  parts  of  this  belt,  particularly  over 
continents  at  a  distance  from  the  oceans,  where  the  continental  indraft  of  the 
warm  season  draws  damp  air  from  the  seas  and  where  the  high  temperature 
then  prevalent  provokes  local  convectional  storms,  the  rainfall  is  greater  in 
summer  than  in  winter.  Thus  the  contrast  between  the  greater  winter  rainfall 
of  Oregon  and  Washington  (state)  in  winter  and  the  greater  summer  rainfall 
of  the  upper  Mississippi  valley  is  explained.  A  similar  contrast  is  found 
between  the  rainfall  of  the  western  coast  of  Europe  and  of  the  interior  plains 
of  Russia  and  western  Asia. 

Kainfall  is  generally  of  sufficient  amount  in  the  belt  of  westerly  winds 
over  the  oceans  as  well  as  over  a  great  part  of  the  lands,  varying  from  thirty 
to  eighty  or  more  inches.  Exception  must  however  be  made  of  continental 
interiors,  distant  from  the  oceans  or  enclosed  by  high  mountains,  where 
these  middle  latitudes  are  arid ;  and  of  bold  western  coasts  of  higher  latitudes, 
where  the  rainfall  becomes  excessive,  reaching  more  than  a  hundred  inches  at 
points  where  the  form  of  the  rising  land  gathers  in  the  wind  and  locally 
increases  the  precipitation.  Even  so  moderate  a  relief  as  that  of  Great 
Britain  shows  a  decidedly  greater  rainfall  on  its  western  slopes,  where  the 
moist  winds  from  the  Atlantic  first  meet  the  highlands,  than  on  the  lower 
eastern  slopes,  where  the  winds  flow  after  having  lost  some  of  their  vapor. 
The  Scandinavian  peninsula  shows  the  same  contrast  more  distinctly ;  and 
it  is  exhibited  with  extreme  emphasis  in  our  western  territories,  of  whir-h 
more  below.  It  must  however  be  borne  in  mind  that  as  the  westerly  winds 
often  blow  over  mountains  without  yielding  rainfall,  while  the  cyclonic 
storms  within  these  winds  give  forth  rain  not  only  on  mountains  but  on  low- 
lands also,  the  storms  and  not  the  action  of  the  mountains  on  tho  gowrsil  winds 
must  be  regarded  as  the  controlling  cause  of  precipitation  in  this  belt ;  while 


THE    CAUSES    AND    DISTRIBUTION    OF    RAINFALL.  301 

the  mountains  serve   locally  to  increase  the  precipitation  that  the  storms 
produce. 

It  is  chiefly  to  cyclonic  storms  that  the  ample  rainfall  of  the  eastern 
United  States  is  due.  There  is  a  rainfall  of  from  30  to  60  inches  or  more 
from  the  96°  or  98°  meridian  to  the  Atlantic  coast,  distributed  with  remarkable 
uniformity  over  this  great  region,  especially  in  the  growing  season,  and  well 
apportioned  through  the  year.  Decidedly  the  greater  part  of  this  comes  from 
cyclonic  storms.  The  amount  increases  towards  the  Gulf  of  Mexico  and 
the  Atlantic  coast,  whence  nearly  all  the  supply  of  vapor  for  this  rainfall 
is  derived.  In  the  Mississippi  valley,  there  is  a  certain  excess  of  the  summer 
fall  over  that  of  the  winter,  mostly  the  product  of  local  convectional 
storms,  whose  opportunity  is  found  chiefly  in  a  certain  part  of  the  cyclonic 
area  ;  but  this  is  on  the  whole  an  advantage  to  agriculture.  Although  droughts 
sometimes  afflict  considerable  districts,  and  floods  occasionally  devastate  the 
larger  valleys,  yet  the  world  hardly  contains  as  large  an  area  as  this  so  well 
adapted  to  civilized  occupation.  The  importance  of  the  warm  waters  of  the 
Gulf  of  Mexico  and  of  the  western  part  of  the  North  Atlantic  eddy  (including 
the  Gulf  Stream)  cannot  be  overestimated  in  this  respect.  Instead  of  our  having 
an  American  Sahara  to  the  south  of  us  in  the  trade  wind  latitudes,  we  have  a 
great  re-entrant  of  the  oceanic  shore  line,  into  which  flows  a  strong  branch 
from  the  vast  equatorial  current  of  warm  waters.  The  general  winds,  turned 
into  an  imperfect  eddy  around  the  North  Atlantic  basin  (Sect.  157),  here  gather 
abundant  vapor  and  shed  it  in  a  beneficent  rainfall  over  the  eastern  half 
of  our  country.  No  formidable  mountain  range  drains  the  winds  of  their 
moisture  on  their  way  inland,  leaving  the  region  to  leeward  a  desert,  as  in 
southern  Asia.  Had  North  America  been  broad  in  the  trade  wind  belt  and 
narrow  further  north,  its  value  as  a  home  for  man  would  have  been  greatly 
diminished. 

297.  Arid  regions  of  the  westerly  winds.  The  southern  part  of  South 
America  is  the  only  considerable  land  area  in  the  belt  of  westerly  winds  in  the 
southern  hemisphere ;  its  narrow  western  slope  has  abundant  rains,  while  its 
broad  eastern  plains  are  comparatively  dry  ;  but  being  for  the  most  part  open 
to  the  adjacent  Atlantic,  they  have  a  small  or  moderate  rainfall  from  passing 
cyclonic  storms.  In  the  northern  hemisphere,  on  the  other  hand,  the  continents 
expand  to  their  greatest  breadth  in  the  latitude  of  the  westerly  winds,  and 
include  arid  or  desert  regions  of  vast  extent.  The  greatest  of  these  extends 
over  the  western  plains  or  steppes  and  the  central  basin  of  Asia.  The  steppes 
lie  so  far  to  the  leeward  of  the  Atlantic  that  the  greater  part  of  the  vapor 
brought  from  that  ocean  has  been  condensed  on  the  way,  and  the  remainder  is 
not  easily  prompted  to  fall.  The  interior  basin,  hemmed  in  on  all  sides  by 
lofty  mountains,  is  an  extremely  arid  region  ;  the  rivers  descending  the  interior 


302  ELEMENTARY  METEOROLOGY. 

slopes  from  the  snowy  ranges  weaken  as  they  emerge  on  the  piedmont  plains 
and  disappear  further  on  in  the  sands  of  the  desert. 

In  North  America,  the  close  approach  of  our  Cordilleras  to  the  Pacific, 
whence  the  westerly  winds  bring  their  vapor,  leaves  a  large  interior  region 
with  deficient  rainfall.  While  the  higher  mountain  crests  and  plateaus  receive 
a  relatively  plentiful  rainfall,  bearing  heavy  forests  above  7,000  or  8,000  feet 
altitude  up  to  the  tree  line  (about  10,000  feet),  the  plains  between  them  are 
for  the  most  part  extremely  dry  and  barren,  and  agriculture  is  limited  to 
localities  where  irrigation  from  mountain  streams  can  be  introduced  without 
too  great  expense.  The  driest  part  of  this  interior  region  lies  in  Arizona  and 
in  the  part  of  southern  California  east  of  the  higher  mountains  ;  here  the 
rainfall  averages  less  than  three  inches  a  year  at  several  stations.  Further 
eastward  and  northward,  the  rainfall  gradually  increases  ;  but  the  influence 
of  the  Cordilleran  rain  shadow  is  felt  half  way  across  our  continent. 

298.  Contrast  of  torrid  and  temperate  rainfalls.  A  marked  contrast  is 
found  between  the  distribution  of  rainfall  under  the  easterly  trades  of  the 
torrid  zone  and  under  the  stormy  westerly  winds  of  the  temperate  zone.  In 
the  former,  the  occurrence  of  cyclonic  storms  is  a  minor  feature,  and  rainfall 
is  prompted  chiefly  by  local  storms  or  by  the  mountain  ranges  on  the  path  of 
the  winds.  In  the  latter,  cyclonic  storms  are  the  rule,  especially  in  winter ; 
mountain  ranges  are  truly  important  in  determining  localities  of  greater  and 
less  rainfall,  yet  cyclonic  disturbances  are  the  chief  rain  makers.  This  is  best 
seen  in  contrasting  the  prevalent  fair  weather  of  the  trade  belts  in  the  broad 
Pacific  with  the  inhospitable  stormy  weather  of  the  high  southern  latitudes, 
where  the  westerly  winds  encircle  the  earth  with  hardly  an  interruption.  The 
few  expeditions  that  have  penetrated  that  forlorn  region  bring  reports  of  its 
continuous  succession  of  blustering  storms,  with  clouds  and  rain  or  snow  ;  the 
unhappy  product  of  an  excessively  large  ocean  surface,  comparable  in  its 
depressing  effects  only  with  the  arid  interior  of  Asia,  where  the  land  area 
is  excessive. 

A  further  contrast  is  found  between  the  rainfall  of  torrid  and  temperate 
latitudes  in  the  occurrence  of  heavy  rainfalls  generally  on  the  eastern  coasts  or 
mountain  slopes  of  the  former,  and  on  the  western  coasts  or  slopes  of  the 
latter.  Torrid  western  coasts  are  wet  chiefly  where  they  receive  the  equatorial 
rains,  as  in  the  Gulf  of  Guinea  and  on  the  Pacific  slope  of  Colombia,  South 
America  ;  both  of  these  regions  lying  somewhat  north  of  the  equator  on 
account  of  the  unsymmetrical  position  of  the  heat  equator.  In  South  Am<'i  ic,i. 
the  eastern  coast  in  Guiana  and  Brazil  and  the  eastern  slope  of  the  torrid 
Andes  are  well  watered ;  while  the  western  slope  in  Peru  and  northern  Chile 
is  dry.  A  similar  arrangement  is  exhibited  in  Africa  south  of  the  equator,  but 
to  the  north,  the  continuity  of  land  towards  Asia  prevents  its  occurrence.  On 


THE   CAUSES   AND   DISTRIBUTION   OF    RAINFALL.  303 

the  other  hand,  British  Columbia  and  Alaska  in  the  new  world  and  the  western 
coast  of  northern  Europe  in  the  old  world  are  the  regions  of  heaviest  rainfall  in 
the  north  temperate  zone ;  while  less  rain  falls  to  the  eastward.  Similarly, 
1'atagonia  has  abundant  rainfall  on  the  side  towards  the  Pacific,  but  is  drier 
towards  the  Atlantic.  Even  in  the  Australian  continent,  the  same  relation 
appears  ;  the  Australian  Alps  having  the  most  rain  on  the  southeastern  slope  ; 
while  in  Tasmania  as  well  as  in  the  islands  of  New  Zealand  the  rainfall  is 
received  chiefly  on  the  western  slope. 

An  exception  to  the  general  arrangement  of  rainfall  in  the  torrid  zone,  as 
stated  above,  is  found  in  India  and  in  the  Malay  peninsula.  There  the  peculiar 
regime  of  the  monsoons  causes  the  western,  not  the  eastern,  coasts  to  receive 
the  heaviest  rain ;  and  the  double  season  of  cyclones  gives  a  larger  share  of 
cyclonic  rainfall  than  is  known  elsewhere  within  the  tropics,  as  is  further 
explained  below. 

299.  Rainfall  of  high  latitudes.     In  passing  further  towards  the  poles, 
the  proportion  of  snow  in  the  total  rainfall  increases,  but  the  total  precipitation 
decreases,  and  in  the  polar  regions  it  is  comparatively  moderate  as  far  as 
observation  goes.     The  annual  sum  is  generally  less  than  fifteen  inches,  and 
in  certain  polar  regions  it  is  less  than  ten  inches.     This  is  to  be  explained 
partly  by  the  absence  of  local  convectional  storms,  on  which  so  much  rain  in 
the  torrid  zone  and  in  the  summer  season  of  the  temperate  zones  depends ;  and 
still  more  by  the  slow  decrease  of  the  capacity  for  vapor  at  the  low  tempera- 
tures of  the  polar  regions ;    so  that  in  spite  of  active  cyclonic  storms,  the 
precipitation  that  they  can  yield  is  of  small  amount.     A  mass  of  saturated  air 
cooled  from  90°  to  80°,  as  might  happen  in  an  equatorial  thunder  storm,  would 
yield  twenty  times  as  much  rainfall  as  if  it  were  cooled  from  0°  to  — 10°  in  a 
polar  cyclonic  storm.     Mention  has  already  been  made  of  the  absence  of  snow- 
flakes  when  the  precipitation  of  the  polar  regions  takes  place  at  temperatures 
urider  5°  or  10°  below  zero ;  the  precipitation  then  is  in  the  form  of  fine  ice 
spicules. 

300.  Migration  of  rain  belts.     The  terrestrial  wind  system  shifts  north 
and  south  in  an  annual  period  following  the  passage  of  the  sun.     The  belt  of 
equatorial  rains  therefore  moves  north  and  south  with  the  doldrums,  as  already 
illustrated  by  Fig.  59,  for  the  torrid  Atlantic,  taken  from  the  Pilot  Charts  of 
the  North  Atlantic,  published  by  our  Hydrographic  Office  at  Washington,  and 
from  other  sources. 

At  certain  stations  near  the  equator,  the  migration  of  the  doldrums  produces 
two  rainy  seasons  with  the  sun  overhead,  and  two  intervening  dry  seasons  when 
the  sun  is  to  the  north  or  south.  This  is  shown  for  several  stations  in  the 
following  table  ;  Sao  Thome  being  an  island  in  the  Gulf  of  Guinea  off  Africa ; 


304  ELEMENTARY   METEOROLOGY. 

the  Gaboon  being  a  part  of  the  equatorial  African  coast  near  by  ;  and  Quito 
and  Bogota  being  in  equatorial  South  America. 

301.  Sub-equatorial  rains.  As  a  consequence  of  this  migration,  each 
trade  wind  belt  is  annually  encroached  upon  by  the  equatorial  rains  as  the 
sun  enters  its  hemisphere.  Thus  the  luxuriant  forests  of  equatorial  Africa 
merge  into  the  wastes  of  the  sandy  Sahara  through  the  more  habitable  belt  of 
the  Soudan,  with  dry  winters  and  wet  summers  ;  the  rainfall  being  less  and  its 
duration  becoming  briefer  as  the  permanent  desert  area  is  approached,  where 
practically  no  rain  falls.  South  of  the  equator  there  is  a  corresponding 
arrangement  of  wet  and  dry  seasons.  A  matter  so  important  to  ancient 
civilization  as  the  flooding  of  the  Nile  depends  on  this  control  of  the  seasonal 
distribution  of  rainfall :  when  the  sun  is  south,  the  Blue  Nile  rising  in  the 
mountains  of  Abyssinia  is  almost  dry  ;  but  as  the  sun  comes  north  and  the 
calms  and  the  rains  follow  it,  the  river  rises  and  its  volume  is  added  to  the 
more  constant  supply  of  the  White  Nile,  which  comes  from  the  great  lakes  of 
equatorial  Africa  ;  thus  causing  the  summer  flood  of  the  trunk  river  in  middle 
and  lower  Egypt. 

In  South  America,  the  extended  plains  or  llanos  of  Venezuela  are  well 
watered  from  May  to  October  when  reached  by  the  equatorial  rains,  but  are 
dry  and  parched  for  the  rest  of  the  year,  when  they  are  swept  over  by  the 
cloudless  trade  wind.  While  the  llanos  are  dry,  the  interior  campos  of  Brazil 
have  their  rains  from  October  to  April,  and  for  the  rest  of  the  year  they  in 
turn  have  clear  and  dry  weather.  These  may  be  called  the  regions  of  sub- 
equatorial  rains.  f 

The  alternation  of  rainy  and  dry  seasons  is  even  more  apparent  in  the 
monsoon  region  of  India.  When  the  sun  is  south,  the  northeast  or  winter 
monsoon  flows  from  the  mountains  to  the  sea,  and  the  greater  part  of  the  vast 
peninsula  is  dry.  In  the  opposite  season,  when  the  high  temperatures  of  early 
summer  have  shifted  the  belt  of  low  pressure  to  northern  India  and  Persia, 
the  southeast  trade  wind  comes  across  the  equator  and  becomes  the  southwest 
or  summer  monsoon,  causing  heavy  rains  on  the  Ghats  or  bold  western  coast 
of  southern  India,  on  the  mountains  of  Biirmah,  and  on  the  lofty  Himalaya 
further  north.  On  the  latter,  the  greatest  rainfall  of  the  world  is  found, 
amounting  to  forty  feet  a  year  north  of  the  Bay  of  Bengal,  and  nearly  all  of 
this  falls  in  the  five  months  from  May  to  September  (see  Sect.  284).  Tl it- 
southeastern  coast  of  Asia  has  for  the  same  reason  a  contrast  between  w»-t 
summers  and  drier  winters,  but  the  difference  is  less  marked  than  in  India. 
Northern  Australia,  with  a  northwest  landward  monsoon  in  the  southern 
summer,  has  its  rainy  season  from  November  to  March  or  April. 

The  following  table  gives  the  precipitation  of  certain  torrid  stations  in 
inches  to  illustrate  the  amount  and  distribution  of  equatorial  and  tropical 
rainfall.  The  pages  in  the  first  column  refer  to  Hann's 


THE    CAUSES   AND    DISTRIBUTION    OF    RAINFALL. 


305 


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306  ELEMENTARY   METEOROLOGY. 

302.  Sub-tropical  rainfall.  The  tropical  belts  of  high  pressure  have  been 
described  as  shifting  towards  and  from  the  equator  with  the  migration  of 
the  heat  equator  and  with  the  increase  and  decrease  in  the  strength  of  the 
circumpolar  whirl  in  winter  and  summer.  With  us,  the  belt  over  which  they 
migrate  is  therefore  occupied  by  the  drying  trade  winds  when  the  sun  is  north 
and  the  stormy  westerlies  when  it  is  south.  The  limits  of  the  subordinate 
belts  thus  defined  are  not  sharply  marked,  yet  the  belts  are  easily  recognized 
by  the  control  that  they  exert  on  the  seasonal  distribution  of  rainfall. 
When  the  calms  or  the  trades  are  present,  the  rainfall  is  small ;  when  the 
westerlies  prevail  in  winter,  rain  falls  during  the  passage  of  their  cyclonic 
storms.  The  belts  in  which  the  rainfall  is  thus  determined  are  called  regions 
of  sub-tropical  rains. 

The  rainfall  of  the  Mediterranean  countries  of  southern  Europe  and 
northern  Africa  is  of  this  kind,  particularly  in  Spain  and  Algeria :  in  the 
summer  season,  they  are  prevailingly  dry  ;  in  winter  they  have  a  fair  share  of 
rain.  In  the  Sahara,  the  southern  limit  of  the  sub-tropical  rains  from  the 
winter  cyclonic  storms  of  the  westerly  winds  approaches  the  northern  limit 
of  the  sub-equatorial  rains ;  hence  only  a  small  part,  if  any,  of  that  desert  is 
truly  rainless.  Southern  Australia  exhibits  the  same  alternation  of  sub-tropical 
seasons,  its  summers  being  controlled  by  the  trade  winds,  and  its  winters  by 
the  margin  of  the  stormy  westerlies.  The  southwestern  extremity  of  Africa 
has  the  same  succession  of  dry  and  wet  seasons. 

On  the  western  side  of  the  American  continent,  the  areas  of  sub-tropical  or 
winter  rains  are  very  distinct,  on  account  of  the  meridional  trend  of  the 
Cordilleras  by  which  the  general  winds  are  turned  more  or  less  completely 
into  eddies  around  the  oceans,  as  explained  in  Section  157.  As  the  Pacific 
westerlies  approach  the  continental  barriers  they  give  off  a  branch  which  turns 
equatorward  to  join  the  trades,  and  here  the  coast  is  dry  in  both  hemispheres ; 
while  that  part  of  the  coast  on  which  the  westerlies  impinge  is  well  watered 
by  their  frequent  storms.  The  indefinite  division  between  these  two  members 
of  the  wind  system  migrates  towards  and  from  the  equator  with  the  shifting 
of  the  tropical  belt  of  high  pressure.  The  western  coast  of  North  America 
may  therefore  be  divided  into  three  districts  with  regard  to  rainfall ;  a 
northern  district,  including  Oregon,  Washington,  British  Columbia  and  Alaska, 
on  which  rains  occur  in  all  seasons,  but  are  heaviest  in  winter  when  the  lands 
are  cold  and  the  westerly  winds  and  their  storms  are  most  active;  a  middle 
district,  in  which  rains  occur  in  winter  when  it  is  occupied  by  the  westerly 
-t  <>i  in-bringing  winds,  but  in  which  there  is  little  or  no  rain  in  summer  when 
it  is  occupied  by  the  warming  branch  winds  toward  the  trades  ;  this  being  the 
condition  of  a  greater  part  of  our  Californian  const  ;  and  a  third  district, 
including  that  part  of  the  coast  over  which  the  warming  branch  winds  and  the 
trades  blow  persistently  through  the  year,  as  is  the  case  in  the  peninsula  <>i 


THE    CAUSES    AND    DISTRIBUTION   OF    RAINFALL. 


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308  ELEMENTARY    METEOROLOGY. 

Lower  California,  where  very  little  rain  falls  at  any  time.  We  find  in  this 
the  full  explanation  of  the  seasonal  distribution  and  the  increase  of  rainfall 
northward  along  the  Pacific  coast ;  from  12  inches  at  San  Diego,  to  24  at  San 
Francisco,  83  at  the  mouth  of  the  Columbia  river  and  105  at  Neah  Bay, 
Washington. 

A  similar  arrangement  of  seasonal  rainfall  is  found  in  Chile.  The  southern 
extremity  of  that  country,  commonly  known  as  the  western  slope  of  Patagonia, 
is  rainy  throughout  the  year.  Northern  Chile  is  persistently  dry,  forming  the 
desert  -of  Atacama.  An  intermediate  portion  is  well  watered  in  winter,  when 
the  stormy  overturnings  of  the  westerly  winds  reach  it,  and  dry  in  summer, 
when  it  is  brushed  over  by  the  warming  branch  winds  from  the  westerlies  to 
the  trades. 

Northern  India  is  peculiarly  situated  with  respect  to  the  sub-tropical  rains. 
In  the  winter  season,  rains  from  weak  cyclonic  storms  occur  on  the  northern 
plains  and  along  the  marginal  ranges  of  the  Himalaya,  progressing  from  west 
to  east,  after  the  usual  fashion  of  the  cyclonic  storms  of  the  temperate  zone, 
and  thus  indicating  their  correspondence  with  the  winter  rains  of  other 
sub-tropical  regions.  But  the  opposite  or  summer  season,  instead  of  being  dry, 
is  wetter  than  the  winter  season,  because  at  this  time  rain  falls  from  the 
storms  of  the  summer  monsoon. 

A  review  of  the  preceding  paragraphs  may  be  made  in  the  table  on  the 
preceding  page,  in  which  several  characteristic  rainfalls  of  the  sub-tropical  belt 
and  of  the  westerly  winds  are  assembled.  The  examples  for  the  United  States 
are  supplied  by  the  national  Weather  Bureau  ;  most  of  the  rest  are  selected 
from  Harm's  Klimatologie,  in  which  many  other  records  may  be  found. 

303.  Effect  of  clouds  and  rainfall  on  the  general  circulation  of  the 
atmosphere.  Section  199  has  explained  the  greater  altitude  reached  by 
cloudy  than  by  clear  local  convectional  currents.  An  extension  of  the  same 
principle  will  explain  the  assistance  given  to  the  general  circulation  of  the 
atmosphere  by  the  liberation  of  latent  heat  in  the  rainy  cloud  belt  around  the 
equator.  The  retarded  cooling  of  the  ascending  currents  in  which  the  clouds 
are  formed  enables  the  air  at  a  given  altitude  to  maintain  a  higher  temperature 
than  it  would  possess  if  clear ;  and  as  the  equatorial  clouds  are  chiefly  formed 
in  the  day-time,  their  presence  arrests  a  certain  share  of  insolation  high  above; 
sea  level.  The  relative  increase  of  temperature  thus  determined  causes  the 
isobaric  surfaces  to  diverge  more  strongly  towards  the  equator  than  they  other- 
wise would,  and  thus  hastens  the  general  circulation.  The  absence  of  high 
temperatures  in  the  lower  air  over  the  torrid  seas  is  in  this  way  in  part  made 
good,  as  far  as  the  terrestrial  winds  are  concerned. 

The  abundant  cloudiness  and  frequent  rainfall  of  high  latitudes  does  not 
counter-balance  the  effect  produced  by  the  rain  clouds  around  the  equator ;  for 


THE   CAUSES   AND   DISTRIBUTION   OF   RAINFALL.  309 

at  the  low  temperatures  prevalent  far  north,  and  south  the  latent  heat  liberated 
is  not  great,  and  the  cooling  of  the  air  is  but  slightly  retarded.  Furthermore, 
as  terrestrial  radiation  is  relatively  strong  in  high  latitudes,  the  presence 
of  clouds  there  must  allow  the  air  to  cool  more  than  if  it  were  clear,  and  hence 
tin-  isobaric*  surfaces  will  converge  poleward  more  rapidly  than  they  would 
otherwise.  It  therefore  seems,  on  the  whole,  that  cloud-making  and  rainfall 
must  accelerate  the  terrestrial  circulation. 

The  co-operation  of  sea  and  valley  breezes  has  been  already  mentioned. 
It  may  now  be  perceived  that  the  clouds  formed  and  the  rain  falling  in  the 
day-time  over  mountainous  islands  must  still  further  aid  the  flow  of  the 
diurnal  winds.  In  the  same  way,  the  heavy  monsoon  clouds  and  rains  of 
India  in  summer  must  help  along  the  inflowing  monsoon  winds.  The  circula- 
tion around  the  low  pressure  areas  of  the  northern  Atlantic  and  Pacific  oceans 
would,  on  the  other  hand,  appear  to  be  retarded  in  winter  by  the  radiation 
from  their  extensive  cloud  masses,  whose  action  in  this  respect  must  tend  to 
diminish  the  excess  of  temperature  that  would  otherwise  prevail  on  account  of 
the  abnormal  warmth  of  the  ocean  waters  and  the  liberation  of  latent  heat 
during  the  formation  of  the  abundant  clouds  of  these  regions. 


ELEMENTARY    METEOROLOGY. 


CHAPTER   XIII. 

WEATHER. 

304.  Weather.     Thus  far,  our  attention  has  been  directed  to  the  study  of 
phenomena  in  their  complete  and  ideal  development ;  the  general  distribution 
of  temperature  over  the  world  ;  the  terrestrial  and  continental  circulations  of 
the  atmosphere  ;  the  larger  and  smaller  storms  ;  all  these  have  been  described 
according  to  their  conditions  of  occurrence,  following  each  one  over  all  the 
area  that  it  occupies.     Similar  phenomena  in  different  parts  of  the  world  have 
been  considered  together  ;  the  low  pressure  at  the  south  pole  with  that  at  the 
north  pole  ;  the  trade  wind  belts  all  around  the  torrid  zone  ;  the  continental 
winds  of  Australia  with  those  of  North  America  ;  the  cyclones  of  the  various 
tropical  oceans  ;  and  so  on. 

In  this  chapter,  we  proceed  in  an  entirely  different  order,  and  consider 
the  actual  succession  of  phenomena,  however  varied  and  arbitrary,  as  they 
are  experienced  at  one  place  or  another  in  different  parts  of  the  world.  Some 
suggestion  of  special  effects  of  this  kind  has  been  presented  in  the  accounts 
of  the  passage  of  a  tropical  cyclone  (Sect.  218),  of  the  weather  changes  caused 
by  the  passage  of  cyclones  in  our  latitudes  (Sect.  243),  and  of  the  passage  of 
thunder  storms  (Sect.  252);  but  these  sections  had  for  their  first  intention 
the  better  understanding  of  the  phenomena  that  they  considered,  rather  than 
the  part  that  these  phenomena  play  in  determining  the  succession  of  events 
that  an  observer  at  any  single  station  would  experience.  We  therefore  now 
turn  more  particularly  to  the  study  of  events  in  the  natural  order  in  which 
they  happen  at  any  single  place  ;  the  successive  meteorological  conditions  of 
all  kinds  being  collectively  named  by  the  term,  Weather. 

305.  Weather  elements.    All  the  atmospheric  conditions  which  an  observer 
notices  by  sight  or  feeling  may  be  called  weather  elements.     These  include 
the  temperature,  humidity  and  movement  of  the  air  about  him,  the  condition 
of  the  sky  as  to  clouds  or  haze,  and  the  occurrence  of  precipitation,  as  rain, 
snow  or  hail.     Inasmuch  as  observations  are  commonly  carried  on  near  sea 
level,  it  follows  that  weather  is  largely  concerned  with  the  conditions  of  the 
lower  air  ;  but  the  weather  noted  by  an  observer  on  a  high  mountain  peak 
would  be  governed  by  the  lofty  currents.     Besides  the  familiar  and  sensible- 
elements  above  named,  it  is  convenient  to  include  another,  of  which  our  unaided 
senses  give  us  no  indication.      This  is  the  pressure  of  the  atmosphere,  as 
determined  by  the  barometer.    Although  not  properly  a  weather  element  itself, 
it  is  of  so  much  importance  in  understanding  the   relations    >f  the   nctml 


WEATHER.  311 

weather   elements  and  the  meaning  of  their  changes,  that  it  is  advisably 
considered  with  them. 

306.  Control  of  weather  changes.  The  change  of  weather  from  one  con- 
dition to  another  may  be  recorded  by  its  effects  on  the  various  instruments 
already  described  for  determining  temperature,  wind,  and  so  on.  This  may 
be  done  without  reference  to  causes  and  without  any  search  for  explanation 
of  change  ;  but  it  is  not  the  object  of  this  book  to  encourage  the  keeping 
of  so  insufficient  a  record  as  such  a  one  would  be.  Along  with  the  careful, 
unprejudiced  record  of  the  facts  of  observation,  there  should  always  go  a 
serious  attempt  to  refer  the  facts  to  their  causes,  whether  simple  or  complex. 
It  is  thus  possible  to  gain  an  understanding  of  many  changes,  and  even  to 
anticipate  their  occurrence  ;  but  much  remains  to  be  discovered  in  this  direction. 

The  chief  controls  of  weather  changes  are,  first,  the  diurnal  variation  of 
insolation  from  day  to  night ;  second,  the  annual  change  of  insolation  from 
summer  to  winter  ;  third,  the  passage  of  cyclonic  and  anticyclonic  areas,  with 
their  attendant  smaller  storms  ;  fourth,  the  passage  of  irregular  surges  of 
pressure  and  temperature,  which  appear  to  be  of  longer  period,  larger  area 
and  slower  eastward  movement  than  cyclones  and  anticyclones  are.  The 
diurnal  and  annual  controls  are  regular  in  period  and  comparatively  regular 
in  value  at  any  single  station ;  they  vary  in  intensity  from  place  to  place  in 
accordance  with  such  factors  as  latitude,  form  and  altitude  of  land,  and  distance 
from  the  sea  ;  they  are  relatively  weak  on  the  torrid  oceans,  and  strong  on  the 
inner  lands  of  high  latitudes.  The  other  controls  are  everywhere  more  or  less 
irregular  in  period,  and  they  vary  greatly  in  frequency  and  intensity  in 
different  parts  of  the  globe. 

The  diurnal  change  gives  us  warm  days  and  cool  nights.  The  annual 
change  brings  us  fair,  warm  summer  weather  and  stormy,  cold  winter  weather. 
Cyclonic  and  anticyclonic  changes  interrupt  the  regular  sequence  of  diurnal 
changes  and  introduce  the  chief  irregular  alternations  from  cloudy,  wet  and 
windy  weather  to  fair  weather  with  relatively  dry  and  quiet  air.  It  is  for  the 
most  part  with  these  changes  that  weather  prediction  is  concerned.  Surges 
seem  to  control  spells  of  weather  that  last  one  or  several  weeks,  as  in  spells 
of  unusual  heat  or  cold  at  different  times  of  year.  Successive  surges  sometimes 
follow  tolerably  regular  periods  ;  these  are  easily  recognized  after  their  com- 
pletion ;  they  continue  as  long  as  they  are  not  broken  by  some  irregular  variation 
of  unknown  cause  ;  but  no  one  has  yet  succeeded  in  determining  beforehand 
how  many  surge  periods  may  occur  or  when  they  will  be  broken  into  by  some 
other  cause  of  change  :  hence,  for  the  present,  the  surge  is  not  practically 
utilized  in  forecasting  the  weather.  The  combination  of  all  controls  produces 
an  exceedingly  irregular  sequence  of  weather  changes,  which  in  our  latitude:; 
cannot  at  present  be  foretold  for  more  than  a  few  days  in  advance  at  most. 


312  ELEMENTARY    METEOROLOGY. 

307.  Weather  of   the   torrid  zone.     The  torrid  zone,  embracing  about  a 
half  of  the  earth's  surface,  and  including  a  large  oceanic  area,  is  for  the 
greater  part  characterized  by  a  regular  sequence  of  distinct  diurnal  changes, 
with  a  comparatively  steady  and  regular  change  of  weather  in  passing  from  one 
season  to  the  next ;  and  further  by  small  interference  from  cyclonic  inter- 
ruptions.    One  day  is   much  like  another ;   even   the    small   double    diurnal 
oscillation  of   atmospheric  pressure  is   repeated  day  after  day  with  trifling 
irregularity.     The  diurnal  range  of  temperature  is  remarkably  steady.     The 
winds  over  the  trade  wind  belt  are  much  steadier  in  direction  and  velocity  than 
we  know  them  here.     Clouds  increase  in  the  day-time  and  decrease  at  night. 
When  rain  occurs,  as  in  the  equatorial  belt,  it  manifests  a  distinct   diurnal 
period,  falling  chiefly  late  in  the  day,  while  the  mornings  are  generally  dry. 

A  closer  study  of  the  weather  of  this  vast  zone  suggests  its  division,  first, 
according  to  the  general  wind  system ;  second,  according  to  ocean  and  land 
areas.  We  thus  recognize,  first,  the  two  belts  of  the  steady  trade  winds ; 
second,  the  equatorial  belt,  where  the  weather  in  different  times  of  year 
varies  with  the  change  from  the  wet  to  the  dry  season.  Each  of  these 
divisions  should  then  be  divided  into  continental  and  oceanic  areas,  as  the 
intensity  of  weather  changes  from  day  to  day  and  from  season  to  season  will 
be  found  much  more  pronounced  on  land  than  at  sea. 

308.  The  trade  wind  belts.     In  the  trade  wind  belts  at  sea,  the  constancy 
of  the  winds  and  the  regular  succession  of  the  moderate  diurnal  range  of 
temperature,  day  after  day,  is  hardly  to  be  imagined  by  those  who  know  only 
our  stormy  portion  of  the  temperate  zone.     The  wind  flows  from  the  same 
quarter  and  with  about  the  same  velocity  day  and  night  ;  clouds  form  in  the 
day-time  in  moderate  amount,  and  generally  die  away  in  the  evening,  seldom 
yielding  rain,  except  near  the  doldrums,  or  when  a  cyclone  passes  by  and  takes 
the  control  of  all  weather  changes  into  its  own  hands.     The  cooler  weather  of 
the  year  has  a  temperature  only  eight  or  ten  degrees  lower  than  the  weather 
in  the  warmer  months. 

In  the  trade  belt  on  land,  the  dryness  of  the  air  and  the  increase  in  the 
diurnal  range  of  temperature  and  of  wind  velocity  are  the  most  notable  features. 
In  the  hot  season,  the  heat  at  mid-day  is  extreme,  and  when  accompanied  by 
high  wind,  bearing  clouds  of  dust  and  sand,  it  may  be  fatal  to  human  life. 
Yet  the  nights  even  in  the  hot  season  are  calm  and  relatively  cool.  Travellers 
in  trade  wind  deserts  make  frequent  mention  of  the  discomfort  or  suffering  in 
the  parching  winds  of  day-time  and  the  comparative  comfort  of  the  nights. 
In  the  colder  season,  the  temperature  at  night  falls  to  a  much  lower  degree 
than  we  associate  with  the  torrid  zone  ;  close  to  the  ground,  it  may  approach 
the  freezing  point,  and  thin  sheets  of  water  in  shallow  vessels,  cooling  by 
radiation  and  evaporation,  may  be  frozen  over.  The  equatorial  margin  of  a 


WEATHER.  313 

trade  wind  belt  partakes  of  the  features  of  the  sub-equatorial  belts,  when  the 
ma  roll  of  the  sun  brings  the  rains  upon  it.  Here  the  weather  at  different 
seasons  is  strongly  contrasted  ;  varying  from  the  extremes  of  warm,  dry  winds 
at  one  season,  to  sultry  calms,  broken  by  violent  thunder  storms  and  drenching 
rains  at  another.  In  the  monsoon  region  of  India,  particularly  in  the  northern 
plains  of  the  peninsula,  the  contrast  between  the  weather  of  different  times 
of  year  reaches  an  extreme  for  the  torrid  zone.  The  weather  in  the  cold 
season  is  cool  and  relatively  dry,  occasionally  including  distinct  cyclonic 
changes,  when  an  extra-tropical  cyclone,  moving  eastward,  causes  non-diurnal 
variations  of  wind,  temperature,  cloudiness  and  rain.  The  early  summer 
brings  days  that  are  hot  enough  for  an  equatorial  latitude  ;  and  when  the  wet 
season  succeeds  the  hot  season,  the  dry  heat  is  followed  by  weather  as  sultry 
and  oppressive  as  that  of  the  doldrums. 

309.  The  equatorial  belt.     Between  the  trade  wind  belts,  the  weather  of 
the  doldrums  is  hot,  moist,  cloudy  and  sultry,  with  calms  or  light  breezes  and 
frequent  rains,   returning  day  after  day  with  much   regularity.     When   the 
doldrums  move  away  and  the  trade  winds  follow  them,  the  weather  for  a  time 
becomes  clearer  and  fresher.     The  equatorial  belt  on  land  is  more  pronounced 
in  its  weather  types  than  at  sea.     Its  heat  is  more  intense;  the  storms  of 
its  daily  rains  appear  to  be  more  severe  than  at  sea ;  its  variation  of  weather 
from  one   season  to  another  is  much   more  marked,  owing  to  the  increased 
migration  of  the  wind  belts  ;  but  it  is  still  essentially  diurnal  weather.     An 
account  of   Java,  by  Junghuhn,  calls  especial   attention   to   the   surprising 
regularity  of  the  daily  weather  changes  in  the  interior  of  the  island.      The 
evening  sky  soon  becomes  clear,  and  the  air  relatively  cool  and  damp.     Late  at 
night,  fogs  form  on  the  lowlands.     The  morning  sun  dispels    the    fogs,  but 
cumulus  clouds  soon  form,  particularly  over  the  mountains,  and  a  light  breeze 
springs  up.    Rain  often  falls  on  the  mountains  in  the  afternoon,  but  soon  after 
sunset,  the  clouds  dissolve  away  again,  and  the  series  of  changes  is  begun  once 
more.     All  of  this  is  purely  diurnal  weather  in  its  perfection. 

310,  The  sea  breeze.     On  the   coasts  of   equatorial  lands,  the  distinct 
diurnal  control  of  the  weather  causes  the  sea  breeze  to  attain  an  importance 
that  it  never  reaches  with  us.     The  old  navigator,  Dampier,  described  this 
with  much  appreciation  two  hundred  years  ago:  — 

"  Sea  breezes  do  commonly  rise  in  the  morning  about  nine  a  clock,  sometimes 
sooner,  sometimes  later ;  they  first  approach  the  shore,  so  gently,  as  if  they 
were  afraid  to  come  near  it,  and  ofttimes  they  make  some  faint  breathings, 
and  as  if  not  willing  to  offend,  they  make  a  halt,  and  seem  ready  to  retire.  I 
have  waited  many  a  time  both  ashore  to  receive  the  pleasure,  and  at  sea  to 
take  the  benefit  of  it.  It  comes  in  a  fine  small  black  curie  upon  the  water, 


314  ELEMENTARY    METEOROLOGY. 

when  as  all  the  sea  between  it  and  the  shore  not  yet  reached  by  it,  is  as 
smooth  and  even  as  glass  in  comparison ;  in  half  an  hour's  time  after  it  has 
reached  the  shore  it  fans  pretty  briskly,  and  so  iucreaseth  gradually  till  1U 
a  clock,  then  it  is  commonly  strongest,  and  lasts  so  till  2  or  3  a  very  brisk 
gale ;  about  12  at  noon  it  also  veers  off  to  sea  two  or  three  points,  or  more  in 
very  fair  weather.  After  3  a  clock  it  begins  to  dye  away  again,  and  gradually 
withdraws  its  force  till  all  is  spent,  and  about  5  a  clock,  sooner  or  later, 
according  as  the  weather  is,  it  is  lull'd  asleep,  and  comes  no  more  till  the  next 
morning.  These  winds  are  as  constantly  expected  as  the  day  in  their  proper 
latitudes,  and  seldom  fail  but  in  the  wet  season." 

"  Land-breezes  are  as  remarkable  as  any  winds  that  I  have  yet  treated  of ; 
they  are  quite  contrary  to  the  sea-breezes  ;  for  those  blow  right  from  the  shore, 
but  the  'sea-breeze  right  in  upon  the  shore  ;  and  as  the  sea-breezes  do  blow  in 
the  day  and  rest  in  the  night ;  so  on  the  contrary,  these  do  blow  in  the  night 
and  rest  in  the  day,  and  so  they  do  alternately  succeed  each  other.  For  when 
the  sea-breezes  have  performed  their  offices  of  the  day  by  breathing  on  their 
respective  coasts,  they  in  the  evening  do  either  withdraw  from  the  coast,  or 
lye  down  to  rest ;  then  the  land-winds  whose  office  is  to  breathe  in  the  night, 
moved  by  the  same  order  of  divine  impulse,  do  rouze  out  of  their  private 
recesses  and  gently  fan  the  air  till  the  next  morning ;  and  then  their  task 
ends  and  they  leave  the  stage." 

"  These  land-winds  are  very  cold,  and  though  the  sea-breezes  are  always 
much  stronger,  yet  these  are  colder  by  far.  The  sea-breezes  indeed  are  very 
comfortable  and  refreshing ;  for  the  hottest  time  in  all  the  day  is  about  9,  10 
or  11  a  clock  in  the  morning,  in  the  interval  between  both  breezes,  for  then 
it  is  commonly  calm,  and  then  people  pant  for  breath,  especially  if  it  is  late 
before  the  sea-breez  comes,  but  afterwards  the  breez  allays  the  heat.  However, 
in  the  evening  again  after  the  sea-breez  is  spent,  it  is  very  hot  till  the  land-wind 
springs  up,  which  is  sometimes  not  till  12  a  clock  or  after." 

These  several  paragraphs  suffice  to  show  that  over  large  areas  of  the  torrid 
zone  the  sequence  of  weather  changes  is  so  simple  and  so  regular,  and  one 
day  is  so  much  like  its  neighbors,  that  almost  any  day  is  a  fair  sample  of  its 
season  ;  and  hence  the  average  of  the  successive  values  of  the  weather  elements, 
or  climate,  hardly  differs  from  the  observed  values  of  individual  occurrences, 
or  weather.  This  will  be  even  more  apparent  in  the  next  chapter,  when  it  is 
seen  how  much  of  what  is  here  said  might  there  be  pertinently  repeated.  1 1 
is  on  the  other  hand  to  be  expected  that  when  closer  attention  is  given  to  the 
individual  weather  features  of  the  torrid  zone,  they  may  be  found  to  be  more 
complicated  than  they  are  here  described. 

311.  Weather  of  the  temperate  zones.  The  middle  latitudes  of  the  earth 
are  characterized  chiefly  by  an  irregular  combination  of  periodic  or  diurnal, 


WEATHER.  315 

and  unperiodic  or  cyclonic  and  anticyclonic  weather  changes.  The  quality  of 
weather  produced  by  this  combination  varies  greatly  in  different  seasons  of 
the  year  ;  the  periodic  changes  predominating,  especially  on  land,  during  the 
higher  temperatures  of  summer  and  in  the  clear  air  of  anticyclones  ;  and  the 
irregular  changes  being  in  control  during  the  lower  temperatures  of  winter. 
The  quality  of  weather  varies  also  very  greatly  according  to  the  situation  of 
the  observer  in  the  zone.  Out  of  the  abundance  of  examples  that  may  be 
found,  space  can  be  given  only  to  a  few  of  the  more  peculiar  ones  :  the 
oceanic  areas,  especially  of  the  southern  temperate  zone  ;  the  land  interiors 
and  the  western  and  eastern  coasts  of  the  northern  temperate  zone. 

312.  The  south  temperate  zone.     The  great  temperate  water  zone  of  the 
southern  hemisphere  is  almost  as  regular  in  its  constant  succession  of  unperiodic 
weather  changes  as  the  trade  wind  belts  are  in  their  unchanging  succession 
of  constant  fair-weather  days.     East-bound  vessels  on  the  southern  seas  sail 
day  after  day  before  boisterous  winds,  whose  strength  often  rises  to  a  gale, 
generally  from  some  westerly  quarter,  shifting  between  northerly  and  southerly 
points,  but  seldom  blowing  from  the  east.     The  temperature  is  inclement ;  its 
average  diurnal  changes  are  small,  for  on  the  ocean  day  and  night  are  light 
and  dark,  rather  than  warm  and  cold  ;  but  its  irregular  changes  are  pronounced 
though  not  severe  ;  they  follow  the  shifts  of  the  wind  rather  than  the  rising 
and  setting  of  the  sun.     Cloudy  skies  are  prevalent  and  rain  or  snow  often 
falls,   but  not  in  excessive   amounts.     In  winter,  the  weather  changes  are 
stronger,  more  frequent  and  more  distinctly  cyclonic ;  they  differ  more  in  this 
way  from  the  weather  changes  of  summer  than  in  a  strongly  decreased  tem- 
perature.    The  accounts  already  given  of  cyclonic  winds  explain  that  in  the 
southern  temperate  zone,  the  northerly  winds  bring  milder  temperature,  clouds 
and  rain  ;  while  the  southerly  winds  follow  with  cooler  and  clearing  weather. 
Little  is  known  of  surges  here,  although  it  is  probable  that  they  occur.     As 
in  the  trade  wind  belt  of  the  torrid  zone,  so  in  the  southern  temperate  ocean 
belt,  a  brief  period  of  observation  will  furnish  a  fair  sample  of  weather  for 
the  season ;  but  here  the  weather  period  is  roughly  two  or  three  days,  being 
the  time  required  for  the  passage  of  a  cyclone  and  an  anticyclone,  instead 
of  the  passage  of  a  day  and  a  night. 

313.  The  north  temperate  zone.     The  weather  over  the  broad  oceanic  areas 
of  the  north  temperate  zone  is  much  like  that  of  the  corresponding  southern 
zone  in  varying  chiefly  with  the  procession  of  cyclones  and  anticyclones  by 
which  its  wind,  temperature  and  sky  are  controlled  ;  although  in  summer  the 
strength  of  these  controls  is  much  weakened. 

The  great  continental  interiors  of  the  north  temperate  zone,  such  as  our 
vast  Mississippi  basin,  are  characterized  by  frequent  spells  of  regular  diurnal 


316  ELEMENTARY   METEOROLOGY. 

weather  in  summer,  after  the  fashion  of  the  torrid  zone ;  yet  even  in  this 
season  cyclones  have  a  considerable  effect.  In  winter,  the  diurnal  control 
weakens,  especially  when  the  ground  is  snow  covered,  and  the  cyclonic  control 
strengthens,  so  as  to  determine  nearly  all  the  changes  from  clear  to  cloudy, 
from  warmer  to  colder,  from  calm  to  windy,  from  wet  to  dry.  Examples 
of  the  weather  in  each  season  may  now  be  given. 

314.  Summer  weather  in  the  central  United  States.  The  warm  spells 
of  summer  time  occur  during  the  gradual  advance  of  a  moderate  cyclonic  area 
over  the  upper  Mississippi  valley.  A  light  southerly  wind  —  a  warm  wave, 
or  sirocco  —  prevails  on  moderate  gradients  in  front  of  the  center  of  low 
pressure.  There  is  at  first  a  strong  diurnal  range  of  temperature,  with  a  quick 
warming  in  the  morning  (see  Fig.  10a)  and  five  or  six  hours  of  high  temperature 
during  the  later  half  of  the  day.  As  the  sky  becomes  more  hazy  or  more 
streaked  with  cirrus  clouds,  the  maximum  temperature  reaches  a  higher  and 
higher  degree  each  day,  and  the  nocturnal  cooling  diminishes  ;  the  air  becomes 
sultry  and  oppressive  with  increasing  humidity  ;  the  ground  is  parched  and 
the  wind  drifts  clouds  of  dust  from  all  bare  surfaces  ;  vegetation  is  stifled ; 
men  and  beasts  suffer  greatly  while  at  labor,  and  sunstrokes  are  reported  in 
increasing  numbers  from  the  crowded  cities.  Unless  a  rain  has  recently 
fallen,  the  sky  may  be  nearly  cloudless  all  this  time  ;  but  near  the  culmination 
of  the  warm  wave,  cumulus  clouds  may  grow  to  the  size  of  local  thunder  storms 
in  the  afternoon,  trailing  a  refreshing  rain  beneath  them ;  yet  the  temperature 
rises  again  after  their  passage.  Near  the  center  of  the  cyclonic  area,  there  are 
clouds  and  rain ;  further  south  along  the  trough  of  low  pressure,  there  are 
extended  thunder  storms ;  and  after  these  pass  by,  a  welcome  shift  turns  the 
wind  to  the  west  and  northwest ;  the  temperature  falls  ten  or  twenty  degrees, 
the  air  becomes  fresh  and  pleasant,  and  the  sky  brightens  to  a  clearer,  darker 
blue.  If  the  rainfall  by  which  the  hot  spell  was  terminated  comes  in  the 
afternoon  or  at  night,  as  is  often  the  case,  there  is  active  evaporation  the  next 
morning  under  the  drying  northwest  wind  —  the  cool  wave  of  summer  —  and 
a  slow  rise  of  temperature  to  a  moderate  maximum  late  in  the  afternoon ;  the 
morning  sky  is  early  flecked  with  growing  cumuli,  and  by  noon  it  may  be 
overcast  above  a  brisk  wind  ;  but  the  night  will  be  clear  and  calm  again,  and 
tin-  next  day  will  be  less  cloudy  as  the  ground  dries.  Then  the  temperature 
increases  day  by  day;  and  generally  by  the  third  day  or  sooner,  the  winds 
weaken  on  the  faint  gradients  of  the  anticyclonic .area  which  then  passes  by; 
the  sky  still  being  fair  and  the  range  of  temperature  strong,  giving  warm  days 
and  pleasant  nights.  As  soon  as  the  pressure  begins  to  fall  on  the  western 
side  of  the  anticyclone,  the  wind  swings  around  to  southerly  again,  and 
another  warm  spell  sets  in.  During  the  whole  of  such  a  period  as  this,  the 
diurnal  changes  are  perfectly  apparent,  and  for  a  part  of  the  time  they  are 


WEATHER.  317 

dominant ;  but  they  fall  to  a  moderate  value  about  the  time  of  highest 
temperature,  when  even  the  nights  are  oppressively  warm.  The  character  of 
each  succeeding  day  manifestly  depends  largely  on  its  relation  to  the  con- 
trolling cyclonic  or  anticyclonic  center,  and  the  consequent  behavior  of  the 
cyclonic  or  anticyclonic  winds.  There  are  of  course  many  occasions  when  the 
areas  of  low  and  high  pressure  are  poorly  denned  and  their  effects  are  weak  ; 
and  there  are  many  days,  especially  in  summer,  when  the  distribution  of 
pressure  is  indefinite,  and  it  is  not  possible  to  locate  any  cyclonic  or  anti- 
cyclonic  centers  within  the  eastern  part  of  our  country ;  but  by  far  the  greater 
number  of  days  may  be  characterized  simply  and  accurately  enough  by  refer- 
ring them  to  one  or  another  part  of  the  passing  areas  of  low  or  high  pressure. 

315-  Winter  weather  in  the  central  United  States.  In  winter,  the  cold 
wave  is  the  emphatic  phenomenon.  Beginning  our  record  with  a  spell  of 
clear,  calm,  cold  weather,  the  ground  being  covered  with  snow,  let  us  notice 
when  the  wind  turns  to  a  southerly  source,  springing  up  perhaps  at  night,  and 
thus  checking  the  nocturnal  fall  of  temperature.  The  next  day  the  tempera- 
ture rises  rapidly  and  a  thaw  sets  in  under  warm  sunshine  ;  but  as  the  day 
passes  the  sky  clouds  over,  and  by  afternoon  the  sun  is  lost  behind  a  dull  bank 
of  matted  cirro-stratus.  Yet  the  temperature  continues  to  rise  even  into  the 
night ;  the  air  is  unseasonably  warm  ;  our  heavy  clothes  and  over-heated  houses 
are  oppressive  ;  the  thaw  progresses  and  is  aided  by  a  fall  of  wet  snow  which 
soon  turns  to  rain.  The  next  morning  is  still  rainy  and  dark  under  low  clouds 
with  misty  air  and  a  foggy  ground ;  gradually  the  wind  shifts  to  westerly ; 
the  rain  ceases,  the  clouds  break  in  the  west  or  northwest,  their  edge  drifts 
obliquely  eastward,  revealing  a  pure  blue  sky,  the  wind  strengthens,  and  then 
even  under  bright  sunshine  the  temperature  begins  to  fall ;  the  freezing 
point  is  soon  passed,  the  half-melted  snow  is  frozen  again  into  an  icy  sheet ; 
the  night  is  cloudless,  windy  and  bitter  cold ;  the  cold  continues  through  the 
next  day  with  hardly  any  rise  of  temperature  under  strong  sunshine.  Then 
the  wind  falls  away  at  sunset,  as  the  anticyclonic  area  comes  on,  and  the 
next  morning  our  neighbors  report  the  most  extreme  cold  in  the  valleys  and 
lowlands,  and  more  moderate  temperatures  011  the  hilltops  —  an  anticyclonic 
inversion  of  temperature  often  amounting  to  twenty  or  thirty  degrees. 

An  extraordinary  instance  of  weather  changes  of  the  kind  just  described 
was  recorded  in  New  England  on  Christmas  Eve,  1886.  The  sky  had  been 
cloudy  under  an  uncomfortable  southerly  wind  that  was  flowing  toward  a 
cyclonic  center  on  its  way  down  the  St.  Lawrence.  The  temperature  had  risen 
almost  continuously  from  the  evening  of  the  23d.  During  the  evening  of  the 
24th  there  was  a  heavy  rainfall,  and  about  midnight  the  highest  temperature 
of  the  month  (55°)  was  registered  ;  a  temperature  that  should  have  been  felt, 
according  to  diurnal  and  annual  controls,  a  little  after  noon  on  the  first  day  of 


318  ELEMENTARY    METEOROLOGY. 

the  month.  Shortly  after  midnight,  the  wind  quickly  shifted  to  the  west ;  the 
temperature  immediately  began  to  fall,  and  continued  to  fall  almost  steadily 
all  the  next  day  under  a  clearing  sky,  thus  ushering  in  a  strong  cold  wave. 

Changes  of  this  kind,  more  or  less  pronounced,  are  so  rapid  in  their  suc- 
cession that  we  seldom  have  more  than  a  day  or  two  of  uniform  weather  in 
winter.  However  fair  the  morning,  the  sky  may  be  half  overcast  with  cirrus 
streamers  by  noon  ;  and  after  sunset  rain  or  snow  may  be  falling.  However 
low  the  clouds  at  one  hour,  twelve  hours  later  may  see  them  all  dispelled. 
The  variety  in  the  succession  of  weather  elements  is  endless  ;  yet  all  the 
variations  are  on  one  theme. 

316.  Weather  of  the  frigid  zones.     During  much  of  the  year  in  high 
latitudes,  the  diurnal  control  of  weather  is  wanting.     Near  the  equinoxes, 
when  the  sun  rises  and  sinks  a  few  degrees  above  and  below  the  horizon,  there 
is  a  semblance  of  the  changes  which  we  know  so  well  ;  but  at  other  seasons 
of  continued  night  or  uninterrupted  day,  the  weather  varies  only  through 
stormy  and  fair  spells,  presumably  the  effect  of  passing  cyclones  and  anti- 
cyclones, as  with  us  in  the  temperate  zone.     During  the  continued  daylight, 
which  Arctic  travellers  find  extremely  tiresome,  the  periods  of  weather  change 
are  no  better  defined  than  the  hours  of  work  and  rest.     Fair  weather  continues 
fair  and  bright  until  broken  by  a  storm  ;  stormy  weather  continues  dull,  snowy 
or  wet  until  followed  by  clearing  skies.     In  the  long  winter  night,  storms  of 
wind  and  snow  are  followed  by  spells  of  quiet  air  and  more  extreme  cold  ; 
but  without  more  regular  sequence  than  results  from  the  irregular  periods  of 
cyclonic  passage. 

WEATHER  ^OBSERVATION  AND  PREDICTION. 

317.  Weather   observations.      At   the   more   important   stations   of  the 
national  weather  services,  at  some  of  the  larger  astronomical  observatories, 
and  at  certain  private  observatories,  all  the  weather  elements  are  carefully 
observed,  frequently  with  self-recording  instruments  by  means  of  which  a  full 
account  of  all  changes  is  taken.    At  less  important  stations  of  national  weather 
services,  at  stations  maintained  by  volunteer  observers  of  state  weather  serv- 
ices, or  by  independent  private  observers,  records  of  temperature,  wind,  sky 
and  precipitation  are  taken  more  or  less  completely  once,  twice,  or  three  times 
a  day.     In  all  these  cases  it  generally  happens  that  the  method  of  reduction 
of  observations  by  averaging  them  in  diurnal  or  monthly  periods  obliterates 
to  a  greater  or  less  degree  the  irregular  changes  of  the  weather,  such  as  result 
from  the   passage  of  cyclones  and  imticyolones,  and  gives  undue  emphasis 
to  the  effects  of  diurnal  and  annual  controls.     The  record  and  rodnrtion  of 
weather  observations  would  therefore  be  improved  by  the  introduction  of  a 


WEATHER.  319 

method  in  which  the  cyclonic  and  anticyclonic  weather  elements  shall  be 
distinguished  in  accordance  with  a  natural  and  simple  classification,  and 
reduced  in  such  a  manner  as  shall  indicate  the  prevalence  of  one  kind  of 
weather  or  another  more  distinctly  than  is  now  done  in  the  customary  averages 
and  summaries. 

For  example,  a  curve  of  temperature,  such  as  is  made  by  a  thermograph, 
exhibits  a  general  rise  and  fall  of  cyclonic  period,  in  addition  to  the  more 
rapid  rise  and  fall  of  diurnal  period.  If  a  pair  of  lines  is  drawn  tangent  to 
the  curves  of  diurnal  range,  one  above  and  the  other  below  the  temperature 
curve,  the  space  between  them  may  be  called  the  temperature  belt.  A  curve 
through  the  middle  of  this  belt  will  represent  the  cyclonic  range  of  tempera- 
ture. The  range  is  stronger  and  of  shorter  period  in  winter  than  in  summer ; 
its  form  is  often  unsymmetrical,  showing  a  more  rapid  fall  than  rise.  In 
winter,  the  diurnal  range  is  small  during  the  cyclonic  fall ;  and  larger  during 
the  cyclonic  rise.  In  summer,  the  diurnal  range  is  least  during  the  prevalence 
of  cyclonic  clouds.  The  form  of  the  diurnal  curve  is  distinctly  different  during 
the  prevalence  of  different  winds.  The  facts  here  referred  to,  and  others  of 
similarly  irregular  period,  have  much  to  do  with  determining  the  character 
of  the  weather  that  we  suffer  from  or  enjoy  ;  and  they  deserve  as  careful 
recognition  as  others  which  are  customarily  considered. 

318.  Weather  Bureau  of  the  United  States.  Soon  after  the  whirling  and 
progressive  motions  of  cyclones  were  recognized  and  their  control  over  weather 
changes  was  perceived,  various  projects  were  devised  for  the  prediction  of  the 
unperiodic  weather  changes.  The  essentials  of  all  these  propositions  were  :  — 
the  appointment  of  a  number  of  observers  at  selected  stations  ;  the  uniform 
observation  of  the  weather  elements  at  the  same  moment  of  time  once,  twice, 
or  three  times  a  day  at  all  the  stations  ;  the  telegraphic  transmission  of  the 
observations  to  a  central  station  ;  the  charting  of  the  data  thus  concentrated  ; 
the  forecasting  of  coming  weather  changes  for  different  districts  by  inference 
from  the  charted  weather ;  and  the  distribution  of  the  forecasts  by  telegraph 
to  the  newspapers,  or  to  numerous  stations  where  weather  signals  might  be 
displayed  for  the  information  of  the  public.  Such  a  plan  was  gradually  put  in 
operation  by  Leverrier  in  France  between  1855  and  1863 ;  in  the  Netherlands 
in  1860  ;  and  in  Great  Britain  in  1861. 

In  this  country,  Professor  Cleveland  Abbe,  with  the  aid  of  the  Cincinnati 
Chamber  of  Commerce,  established  a  weather  service  on  a  small  scale  for  the 
Ohio  valley  in  1869.  This  was  followed  in  1870  by  a  much  more  extensive 
scheme,  developed  by  General  Myer,  Chief  Signal  Officer  of  the  Army,  and 
adopted  by  Congress  as  a  national  service.  A  corps  of  weather  observers 
was  then  established  all  over  the  country.  Professor  Abbe  was  retained  as 
scientific  adviser  at  the  central  office  in  Washington,  where  he  was  for  some 


320  ELEMENTARY    METEOROLOGY. 

time  the  only  predicting  officer  on  duty.  The  service  was  greatly  extended 
from  year  to  year  under  General  Myer,  and  after  his  death  by  his  successors, 
Generals  Hazen  and  Greely.  On  July  1,  1891,  the  meteorological  division  of 
the  Signal  Service  was  by  order  of  Congress  transferred  from  the  Army  to 
the  Agricultural  Department,  under  the  title  of  the  Weather  Bureau,  and 
Professor  M.  W.  Harrington  was  appointed  as  its  chief. 

There  were  24  observing  stations  in  1870  ;  in  1893  there  are  136,  besides 
several  stations  of  the  Canadian  Weather  Service  from  which  daily  reports 
are  received.  Observations  were  first  taken  and  synoptic  charts  prepared 
three  times  a  day  ;  but  on  January  1,  1889,  two  observations  a  day,  at  eight 
o'clock,  morning  and  evening,  were  substituted,  As  at  present  developed, 
after  continued  growth  for  over  twenty  years,  the  Weather  Bureau  has  a 
central  office  in  Washington,  with  a  staff  of  predicting  officers  and  a  number 
of  assistants  for  the  discussion  of  its  voluminous  records.  There  are  stations 
in  all  the  larger  cities,  at  which  observations  are  made  systematically  on 
pressure,  temperature,  wind,  humidity,  sky,  precipitation,  etc.  These  observa- 
tions are  corrected  for  instrumental  errors,  and  the  barometric  readings  are 
reduced  to  sea  level  (Sect.  107).  The  records  are  then  translated  into  an 
abbreviated  cipher  and  telegraphed  promptly  to  Washington  ;  the  messages 
having  precedence  over  all  others.  They  are  generally  all  received  in  the 
Washington  office  within  an  hour  of  the  time  of  observation.  The  cipher 
is  then  translated  back  into  the  original  form,  and  the  data  are  charted  on 
a  number  of  maps.  The  predicting  officer  011  duty  for  the  time  then  draws 
in  the  isobars,  isotherms,  lines  of  equal  change  of  temperature,  etc.,  and  at 
once  proceeds  to  prepare  a  written  statement  of  the  probable  condition  of  the 
coming  weather  for  different  districts  of  the  country. 

The  difficulty  of  this  work  is  very  great.  It  must  be  done  rapidly;  an  hour 
and  a  half  or  two  hours  from  the  time  of  observation  must  ordinarily  suffice 
for  its  completion.  The  weather  for  forty-five  divisions  of  the  country  must 
be  determined  separately ;  and  in  every  case  the  predictions  must  cover  a 
definite  number  of  hours ;  ordinarily  twenty-four  but  sometimes  thirty-six 
hours  from  the  time  of  observation.  Besides  the  general  forecasts  for 
publication  in  the  newspapers,  special  predictions  are  often  called  for,  such 
as  warnings  of  frosts,  river  floods,  cold  waves,  off-shore  winds,  or  expected 
dangerous  storms.  Several  of  these  involve  special  instructions  to  stations 
within  a  certain  district  for  the  display  or  withdrawal  of  signal  flags,  in 
accordance  with  which  vessels  will  delay  their  departure  from  various  ports, 
or  set  sail  on  their  voyages.  The  burden  of  these  manifold  duties  on  the 
predicting  officer  is  so  great  that  in  recent  years  the  experiment  has  been  tried 
of  appointing  a  number  of  local  forecast  officials  in  the  chief  cities,  to  the 
number  of  twenty-six,  and  supplying  them  with  telegraphic,  data  sufficient  to 
construct  synoptic  weather  maps  and  prepare  local  forecasts  for  a  neighboring 


WEATHER.  321 

region.  There  does  not,  however,  at  present  appear  to  be  a  great  increase  in 
the  accuracy  of  local  predictions  over  those  for  the  same  period  and  district 
issued  from  the  Washington  office. 

Besides  the  regular  observing  stations  of  the  Weather  Bureau,  there  are 
numerous  stations  on  the  sea  coast  and  the  Great  Lakes,  where  warnings  are 
given  of  approaching  storms.  Weather  flags  and  other  signals  are  displayed 
by  volunteer  assistants  of  the  Bureau  at  numerous  points  in  all  parts  of  the 
country,  for  the  benefit  of  farmers  and  others.  In  addition  to  the  regular 
paid  officials  of  the  Weather  Bureau,  there  are  some  two  thousand  voluntary 
observers  in  different  parts  of  the  country,  who  report  through  the  state 
weather  services  or  direct  by  mail  to  Washington  once  a  month,  and  whose 
records  are  employed  with  those  from  the  regular  stations  in  the  determination 
of  climatic  data.  Voluntary  observers  are  furnished  with  a  pamphlet  of 
instructions,  and  are  in  some  cases  supplied  with  instruments.  At  different 
times,  special  investigations  have  been  undertaken  with  the  assistance  of 
voluntary  observers  ;  thunder  storms  and  tornadoes  being  the  most  generally 
interesting  subjects  studied  in  this  way. 

The  two  daily  weather  maps,  issued  from  the  central  office  in  Washington 
and  from  over  seventy  other  cities  in  different  parts  of  the  country,  are 
supplied  to  persons  who  will  expose  them  conveniently  for  the  information  of 
the  public ;  they  are  also  sent  in  large  numbers  to  schools.  The  daily 
edition  of  these  maps,  outside  of  Washington,  is  about  8,000.  A  Monthly 
Weather  Review  is  published  and  distributed  to  all  cooperating  observers, 
giving  a  great  body  of  information  from  regular  and  voluntary  reports ;  with 
an  account  of  the  storms,  winds,  temperature,  precipitation,  etc.,  of  the  month, 
and  with  charts  of  cyclonic  tracks,  rainfall,  temperature,  and  so  on.  Weather 
crop  bulletins  are  issued  every  week  during  the  growing  season,  giving 
information  of  interest  to  farmers. 

In  addition  to  the  daily  maps  of  the  Weather  Bureau,  mention  should  be 
made  of  the  Monthly  Pilot  Charts  of  the  North  Atlantic  Ocean,  issued  by  the 
Hydrographic  Office  of  the  Navy  Department  at  Washington  primarily  for 
distribution  to  masters  of  vessels  ;  and  containing  a  large  amount  of  general- 
ized and  current  information  concerning  the  winds,  storms,  ice,  etc.,  of  the 
North  Atlantic  from  month  to  month.  All  notable  storms  are  carefully 
followed  between  North  America  and  Europe  by  means  of  reports  from 
vessels ;  and  in  several  cases  especial  accounts  of  them  are  issued  as 
supplements  to  the  Pilot  Charts. 

319.  Weather  maps.  Of  all  the  publications  of  the  Weather  Bureau,  the 
daily  weather  maps  are  of  the  greatest  interest  to  the  student  of  meteorology, 
whether  a  beginner  or  an  advanced  investigator.  If  the  facts  commonly  shown 
on  these  maps  are  not  familiar  from  school  years  earlier  than  those  in  which 


322 


ELEMENTARY    METEOROLOGY. 


this  book  is  employed,  the  following  plan  of  studying  them  may  be  introduced; 
it  being  advisable  that  the  study  of  the  maps  should  proceed  as  a  course  of 
"  laboratory  work  "  parallel  to  the  study  of  the  text. 

The  meaning  of  the  conventional  signs  on  the  map  should  be  first  learned.1 
Practice  in  drawing  isotherms  and  isobars  of  simple  examples  should  be  afforded. 
Verbal  descriptions  of  various  weather  elements  should  then  be  attempted,  as  : 
"  the  temperature  is  low  in  the  northwest  and  high  in  the  southeast,"  or  "  a 
large  cloud  area  covers  the  Ohio  valley,"  and  so  on  ;  each  element  being 
considered  separately.  By  shading  the  areas  of  different  temperatures  or 


FIG.  106. 

pressures,  an  instructive  series  of  maps  may  be  prepared  for  school  use. 
Besides  the  lines  of  equal  temperature  and  pressure,  called  isotherms  and 
isobars  respectively,  lines  of  the  most  rapid  decrease  of  temperature  and 
pressure  (at  right  angles  to  the  isotherms  and  isobars)  should  be  drawn  on 
trial  maps,  and  their  course  described.  The  lines  of  decrease  of  pressure  will 
be  frequently  seen  to  diverge  from  areas  of  high  pressure,  or  to  converge  toward 

1  In  elementary  teaching,  the  reduction  of  barometric  observations  to  sea  level  cannot  be 
explained ;  it  must  suffice  to  say  in  effect :  certain  corrections  are  applied  to  the  observations 
at  different  stations  in  order  to  make  them  comparable. 


WEATHER.  323 

areas  of  low  pressure.  The  rates  at  which  the  temperature  and  pressure  decrease 

along  their  respective  lines  —  the  gradients  of  temperature  and  pressure 

should  be  determined,  and  expressed  numerically  and  verbally.  The  isobars 
and  pressure  gradients  are  drawn  in  Fig.  106  for  8  P.M.,  November  28,  1888, 
the  date  of  a  disastrous  cyclone  on  the  Atlantic  coast. 

As  the  wind  arrows  represent  the  movement  of  the  air  only  at  isolated 
points  of  observation  in  great  atmospheric  currents  that  sweep  broadly  over 
the  country,  additional  wind  lines  should  be  added  between  the  arrows  for  the 
more  graphic  illustration  of  the  presumed  facts,  as  in  Figs.  53  and  54.  The 
greater  or  less  velocity  of  the  winds,  indicated  by  figures  near  the  arrows  on 
the  published  maps,  may  be  graphically  illustrated  by  heavier  or  lighter  wind 
lines.  The  inflow  and  outflow  of  great  eddies  of  wind  —  cyclones  and  anti- 
cyclones —  should  be  discovered,  and  the  prevailing  wind  velocities  for  each 
recognized.  Outline  maps  for  characteristic  weather  types  should  be  prepared, 
like  those  given  in  Figs.  74  to  79. 

After  deliberate  practice  in  describing  the  several  weather  elements  sepa- 
rately, their  correlation  should  be  considered.  The  most  important  correlation 
is  that  between  the  rate  and  direction  of  pressure  decrease,  or  barometric 
gradient,  and  the  velocity  and  direction  of  the  wind.  The  general  deflection 
of  the  wind  to  the  right  of  the  gradients  and  the  increase  of  the  velocity  on 
stronger  gradients  will  soon  be  perceived.  In  more  careful  study,  a  numerical 
relation  may  be  established  between  these  paired  factors.  By  tracing  the  wind 
arrows  from  various  maps  on  a  single  sheet  of  transparent  paper,  whose  center 
is  always  placed  over  the  center  of  low  or  high  pressure,  with  one  side  always 
turned  to  the  north,  a  "  composite  portrait "  of  a  large  number  of  winds  in 
their  proper  relation  to  the  low  or  high  pressure  center  may  be  obtained,  from 
which  their  inward  or  outward  spiral  courses  will  be  at  once  perceived.  The 
distribution  of  humidity,  of  clouds,  and  of  rain  or  snow,  and  the  peculiar 
warping  of  the  isotherms  with  respect  to  areas  of  high  and  low  pressure, 
are  of  equal  importance.  When  it  is  perceived  that  the  winds  flow  obliquely 
towards  a  center  of  low  pressure,  their  escape  upwards  must  be  inferred ; 
and  the  prevalence  of  clouds  and  rainfall  in  such  regions  may  then  be 
associated  with  the  adiabatic  cooling  of  the  obliquely  ascending  currents. 
The  opposite  relation  will  be  inferred  in  areas  of  high  pressure. 

After  perceiving  the  systematic  correlation  of  the  various  weather  elements 
with  the  centers  of  cyclonic  and  anticyclonic  areas,  the  movement  of  the  center 
of  these  areas  may  be  traced  on  the  maps  of  successive  dates ;  their  paths  may 
be  gathered  on  a  single  map  at  the  end  of  a  month,  and  their  average  velocity 
and  direction  determined.  In  as  much  as  the  association  of  weather  elements 
with  these  high  and  low  pressure  centers  is  relatively  independent  of  their 
position  on  the  map,  a  typical  diagram  of  the  weather  around  a  cyclone  or 
anticyclone,  with  its  winds,  temperature  lines,  clouds,  and  so  on,  may  be 


324  ELEMENTARY  METEOROLOGY. 

prepared  on  a  sheet  of  tracing  paper  on  the  same  scale  as  the  maps,  and  by 
moving  this  diagram  across  a  map  along  an  appropriate  track,  the  succession 
of  weather  changes  experienced  at  any  point  on  the  map  beneath  it  may  be 
simply  illustrated.  All  the  phenomena  of  veering  and  backing  winds,  of 
rising  and  falling  temperature,  and  so  on,  may  be  thus  made  as  plain  as 
desired. 

During  this  advance  in  the  study  of  the  weather  maps,  there  should  be 
frequent  local  observations  of  the  weather,  and  the  facts  thus  noted  should 
be  compared  with  the  larger  collection  of  facts  from  many  stations  on  the 
corresponding  weather  maps.  The  correlation  of  local  and  general  weather  will 
thus  be  clearly  perceived,  the  essential  facts  being  discovered  gradually  by  the 
members  of  the  class  rather  than  taught  by  the  teacher.  Practice  in  prediction 
may  then  be  introduced  ;  but  this  should  not  come  too  soon,  for  fear  that 
it  might  deteriorate  into  guess-work.  It  is  not  a  little  interesting  to  remark 
how  easily,  rapidly  and  naturally  a  class  of  school  children  may,  by  means  of 
the  wonderful  collection  of  facts  simply  presented  on  the  weather  maps, 
advance  through  a  study  which  but  a  few  score  of  years  ago  was  open  only  by 
laborious  investigation  to  the  most  distinguished  meteorologists  of  the  world. 

320,  Methods  of  weather  prediction.  The  essential  principle  employed  in 
the  prediction  of  weather  changes  by  means  of  weather  maps  depends  on  the 
general  eastward  movement  of  weather  areas.  If  a  center  of  low  pressure 
is  charted  in  Colorado,  while  an  area  of  high  pressure  stands  over  West 
Virginia,  southerly  winds  and  rising  temperature  will  be  predicted  for  the  area 
occupied  by  northerly  winds  or  calms  east  of  or  within  the  area  of  high  pressure  ; 
rains  will  be  announced  for  the  area  about  Missouri  over  which  clouds  are 
forming  east  of  the  cyclonic  center  ;  and  clearing  weather,  westerly  winds  and 
falling  temperature  will  be  inferred  for  the  cloudy  and  rainy  region  of  the 
cyclone  itself.  Supplementary  to  this  main  rule,  there  is  a  general  tendency 
to  an  increase  in  the  intensity  of  storms  as  they  approach  the  Atlantic  coast. 
Variations  from  the  average  direction  or  velocity  of  progress  of  weather  areas 
are  seldom  accurately  foretold.  When  the  distribution  of  pressure  is  equable 
and  no  distinct  areas  of  high  or  low  pressure  are  contained  within  the  limits  of 
the  map,  every  predicting  officer  may  have  his  own  method,  more  or  less 
con'sciously  defined,  of  determining  the  most  probable  sequence  of  weather. 
At  times  of  decided  departure  of  any  element  from  the  normal,  a  return 
toward  the  normal  is  usually  predicted,  even  if  there  is  no  very  apparent 
reason  for  it :  for  this  purpose,  charts  have  been  published  by  the  Weather 
Bureau  giving  the  normal  values  of  certain  weather  elements  for  different 
stations  and  for  short  intervals  of  time,  such  as  periods  of  ten  days. 

In  reviewing  this  statement,  and  even  while  appreciating  the  great  value 
of  the  predictions,  one  cannot  avoid  a  certain  feeling  of  disappointment  tluit, 


WEATHER.  325 

all  the  labor  that  has  been  bestowed  on  the  subject  of  weather  prediction 
in  this  country  during  the  last  twenty  years  has  not  led  to  greater  advances 
beyond  the  methods  employed  and  the  results  gained  at  the  outset  of  the 
undertaking.  The  number  of  stations  has  grown,  and  their  equipment  has 
been  materially  improved ;  the  accuracy  of  various  processes  preparatory  to 
charting  has  been  increased  ;  a  vast  body  of  information  has  been  accumulated 
for  study  relative  to  the  kinds  and  changes  of  weather  ;  various  predicting 
officers  have  had  extended  practice  in  their  art ;  and  while  the  forecasts  are 
truly  made  for  longer  periods  than  they  were  at  first,  and  are  certainly 
superior  in  definiteness  and  accuracy  to  those  issued  twenty  years  ago,  their 
improvement  is  not  so  great  as  was  hoped  for.  Mistakes  in  prediction  are 
still  made,  and  of  much  the  same  kind  as  at  the  beginning  of  the  service  ;  it  is 
still  often  impossible  to  predict  the  weather  changes  for  more  than  twenty- 
four  hours  in  advance  with  a  desirable  degree  of  certainty.  It  must  be  con- 
cluded from  this  that  the  limit  of  accuracy  of  weather  prediction  by  present 
methods  is  practically  reached;  and  that  no  considerable  advance  over  the 
inherent  difficulties  of  the  subject  can  be  expected  until  some  distinctly 
new  method  of  observing,  charting  or  forecasting  is  introduced. 

321.  Distribution  of  predictions.     On  the  other  hand,  a  decided  advance 
has  been  made  in  the  distribution  of  the  predictions  and  in  their  utilization 
by  the  people.     Besides  a  very  general  publication  of  the  forecasts  in  the  daily 
papers,  with  an  increasing  completeness  of  statement  year  by  year,  there  has 
been  a  remarkable  growth  in  the  number  of  daily  weather  maps  distributed 
from   many  centers,  and  a  gradual  and  general  increase   in   the  number  of 
display  stations  of  different  kinds.     A  generation  of  our  population  has  grown 
up,  accustomed  to  look  for  and  to  use  the  forecasts  of  the  Weather  Bureau. 
Apart  from  the  value  of  the  forecasts  to  the  general  public,  to  whom  they  are 
a  matter  of  convenience  rather  than  a  necessity,  innumerable  examples  of  their 
more  technical  value  might  be  cited.    Coasting  and  fishing  vessels  are  informed 
before  leaving  port  if  they  are  likely  soon  to  meet  a  storm  on  their  course ;  or 
they  are  warned  not  to  sail  if  dangerous  winds  are  immediately  expected ;  and 
this  information  is  particularly  important  to  the  smaller  craft  engaged  in  the 
local  coasting  trade.    Wrecks  are  relieved  by  assistance  summoned  by  messages ' 
from  the  coast  telegraph  stations.     Disasters  on  the  Great  Lakes  have  been 
materially  diminished  by  warnings  of  coming  storms.     Proper  care  is  given  to 
various  crops  at  critical  times  in  their  growth  or  harvesting ;  although  in  this 
direction,  the  farmers  of  our  country  have  yet  much  to  learn. 

322.  Weather  and  storm  signals.     The  flags  now  employed  for  indicating 
the  predicted  weather  are  as  follows  :  —  a  square  white  flag  for  fair  weather ; 
a  square  blue  flag  for  rain  or  snow,  no  distinction  being  made  as  to  amount ; 


326  ELEMENTARY    METEOROLOGY. 

a  triangular  black  flag  for  temperature,  indicating  warmer  weather,  if  displayed 
above  the  white  or  blue  flag ;  colder  weather,  if  below.  The  changes  of 
temperature  thus  indicated  are  always  to  be  understood  as  being  independent 
of  diurnal  changes ;  and  hence  show  an  expected  rise  or  fall  compared  to 
corresponding  hours  on  the  preceding  day.  A  square  white  flag  with  a  square 
black  center  indicates  the  coming  of  a  cold  wave.  This  is  not  displayed 
unless  it  is  expected  that  the  temperature  will  fall  suddenly  and  decidedly 
to  below  42°,  and  generally  to  a  much  lower  temperature;  twenty-four 
hours  or  more  notice  is  generally  given  of  this  change.  Similar  signals  are 
displayed  on  either  side  of  the  baggage  car  on  railroad  trains  in  some  parts 
of  the  country ;  steam  whistles  are  also  employed,  a  system  of  long  and  short 
blasts  sufficing  to  express  the  predicted  changes. 

Storm  signals  displayed  at  ocean  and  lake  ports  are  as  follows  :  —  The 
cautionary  signal,  displayed  only  on  the  Great  Lakes,  is  a  square  red  flag 
with  white  center ;  this  indicates  the  approach  of  winds  not  so  severe  but 
that  sea-worthy  vessels  can  meet  them  without  danger.  The  storm  signal, 
indicating  severe  winds,  is  a  square  red  flag  with  a  black  center.  A  white 
or  red  pennant,  displayed  with  the  cautionary  or  storm  signal,  indicates  the 
direction  of  the  expected  winds  :  a  red  pennant  above  the  square  flag  is  for 
northeasterly  winds ;  below,  for  southeasterly  winds  ;  a  white  pennant  above 
the  square  flag  is  for  northwesterly  winds;  below,  for  southwesterly.  The 
red  pennant  alone  indicates  that  the  local  observer  has  received  information 
from  the  central  office  at  Washington  concerning  a  storm  that  may  prove 
dangerous  to  departing  vessels.  At  night  a  red  light  indicates  easterly  winds  ; 
a  white  light  above  a  red  indicates  westerly  winds. 

Many  instances  might  be  quoted  in  illustration  of  the  manner  in  which 
weather  changes  have  been  heralded  by  predictions  and  signals  in  season  to 
allow  all  possible  preparation  to  be  made  for  their  coming.  Thus,  a  cold 
wave  is  perceived  while  yet  in  its  earlier  stages  as  an  accumulation  of  cold 
air  under  high  pressure  in  Winnipeg,  north  of  an  advancing  cyclonic  center 
in  Texas  or  Arkansas.  The  wave  is  then  announced  as  a  winter  "  norther " 
before  it  reaches  the  Gulf  coast ;  its  further  progress  eastward  is  anticipated 
by  information  sent  to  the  cities  in  the  Mississippi  valley  ;  its  extension  up 
the  Ohio  valley  and  over  the  southern  states  even  as  far  as  Florida  and  to 
the  middle  Atlantic  seaboard  is  fully  predicted.  Stock  raisers  on  the  western 
plains,  railroad  employees  in  charge  of  cattle  trains,  beef  companies  and  pork 
packers  in  the  central  states,  shippers  of  perishable  goods,  ice  companies 
waiting  to  gather  their  winter  crops  on  the  northern  lakes  or  rivers,  and 
orange  growers  in  Florida,  all  prepare  for  the  best  or  worst  that  the  cold 
wave  may  bring  them.  The  janitors  in  large  buildings  in  our  cities  strengthen 
their  fires  when  the  cold-wave  flag  is  displayed,  and  the  insurance  patrols 
redouble  their  vigilance  in  anticipation  of  conflagrations  so  often  caused  by 


WEATHER.  327 

overheated  chimneys,  and  so  difficult  to  extinguish  when  a  gale  fans  the 
flaiires  and  water  freezes  on  the  walls. 

In  a  similar  manner,  the  arrival  of  a  tropical  hurricane  at  Cuba  is  announced 
by  submarine  telegraph,  and  its  deliberate  but  destructive  advance  along  our 
southern  Atlantic  coast  is  duly  published.  The  late  frosts  of  spring  and  the 
early  frosts  of  fall  are  foretold  to  fruit  farmers ;  cotton  planters  in  the  south 
look  for  warnings  of  wet  or  damp  weather.  The  news  of  the  ending  of  a  hot 
wave  in  summer  by  a  turn  to  westerly  winds,  or  of  the  end  of  a  drought  by 
the  advance  of  rain  storms,  is  watched  for  by  thousands  or  even  by  hundreds 
of  thousands  in  city  and  country.  The  more  intelligent  the  population,  the 
greater  is  the  use  made  of  weather  forecasts. 

Unhappily,  storms  and  frosts  cannot  always  be  correctly  foretold ;  and 
the  changes  that  are  Announced  do  not  always  come  to  pass.  Exceptional 
atmospheric  movements  cannot  be  predicted  by  the  methods  now  employed. 

323.  State  weather  services.  Although  the  records  of  state  weather  serv- 
ices are  useful  chiefly  in  climatic  studies,  yet  their  present  close  association 
with  the  national  Weather  Bureau  suggests  their  description  at  this  place. 
Half  a  century  ago,  systematic  observations  were  undertaken  by  unpaid 
observers  in  Xew  York  and  Pennsylvania ;  but  these  were  discontinued  after 
a  few  years.  The  earliest  state  service  in  recent  years  was  established  by 
Professor  Hinrichs  in  Iowa  in  1873.  At  first  neglected  by  the  national  service, 
the  state  services  were  later  greatly  extended  by  its  aid,  and  since  the  transfer 
of  the  Bureau  to  the  Agricultural  Department,  they  have  been  carried  into 
all  the  states  and  territories.  In  some  cases  they  are  supported  by  small 
sums  of  money -annually  granted  by  the  local  legislatures  ;  they  are  then  well 
equipped  with  uniform  and  accurate  instruments,  and  their  records  are  fully 
published  ;  but  the  work  of  the  observers  is  always  gratuitous.  In  other  cases 
no  state  support  has  been  granted,  and  the  observations  are  less  systematically 
carried  on.  In  all  cases,  a  member  of  the  national  service  is  detailed  to 
supervise  the  work  of  the  local  volunteer  observers  ;  some  form  of  publication 
is  maintained,  and  the  digested  results  of  every  month  are  early  forwarded  to 
Washington  for  use  in  the  preparation  of  the  climatic  tables  and  charts  in  the 
monthly  Weather  Eeview.  The  observations  required  are  all  of  the  simplest 
character,  being  limited  as  a  rule  to  temperature  aud  precipitation  ;  winds 
and  clouds  are  sometimes  included.  Occasionally,  studies  of  subjects  a  little 
aside  from  the  usual  routine  of  weather  observations  have  been  attempted, 
and  it  is  extremely  desirable  that  such  investigations  should  be  extended. 
Even  where  the  most  work  of  this  kind  has  been  done,  there  is  a  wide  field 
open  for  new  or  better  work. 

It  is  very  important  that  the  observers  in  the  state  services  who  are  willing 
to  give  their  time  perseveringly  and  regularly  to  the  long  task  of  determining 


328  ELEMENTARY    METEOROLOGY. 

local  climatic  data,  should  be  equipped  with  good  instruments,  and  not  waste 
their  efforts  in  making  records  whose  accuracy  is  not  proportionate  to  the 
care  bestowed  on  them.  The  moderate  cost  of  good  instruments  in  the  begin- 
ning should  be  somehow  met ;  otherwise  it  is  hardly  worth  while  to  begin 
the  task.  Once  well  begun,  it  should  be  persevered  in  ;  for  the  value  of  local 
records  increases  greatly  as  their  uninterrupted  duration  is  prolonged.  When- 
ever possible,  an  analytical  account  of  local  weather  changes,  a  comparison 
of  local  events  with  the  general  phenomena  of  the  weather  maps,  and  a 
careful  discussion  of  the  results  should  be  attempted,  even  if  only  for  private 
information,  somewhat  as  indicated  in  Section  317. 

324.  Private  meteorological  observatories.     In  a  few  instances,  private 
observatories  have  been  established  for  the  study  of  meteorology.     The  most 
noted  of  these  in  this  country  is  the  Blue  Hill  Observatory,  established  near 
Boston  by  Mr.  A.  L.  Rotch  in  188o.     A  few  years  after  its  foundation,  it  was 
associated  with  the  Astronomical  Observatory  of  Harvard  College,  in  whose 
Annals  its  records  are  elaborately  published.     Notable  among  these  is  a  series 
of  cloud  measurements,  the  most  extensive  yet  undertaken  in  this  country. 
Figs.  65,  66,  68,  69  have  been  prepared  from  these  records  by  the  observer, 
Mr.  H.  H.  Clayton.     The  establishment  of  similar  observatories  in  other  parts 
of  the  country  would  do  much  for  the  advance  of  the  science. 

325.  Foreign   weather   services.      Since   the  establishment  of  the  first 
weather  service  by  Leverrier  in  France,  similar  services  have  been  organized 
by  nearly  all  the   civilized  nations  of  the  world.     The  following  countries 
publish  weather  maps  and  issue  forecasts  :  —  Great  Britain,  France,  Germany, 
Belgium,  Austria-Hungary,  Switzerland,  Italy,  Russia,  Spain,  British  India, 
Japan,  Australia,  and  New  Zealand.     Canada  and  Cape  Colony  issue  forecasts, 
but  do  not   publish  maps.     The  following  countries  maintain  a  system  of 
observations,  and  issue  certain  reports,  but  do  not  prepare  daily  maps  or  issue 
forecasts:  —  Norway,   Sweden,   Denmark,  Netherlands,   Portugal,   Roumania, 
Algeria,  Mexico,  Brazil,  Argentine  Republic,  Chili,  China,  and  certain  smaller 
countries. 

With  the  exception  of  India,  which  lies  largely  in  the  torrid  zone,  the 
features  of  the  weather  maps  of  different  countries  are  astonishingly  uniform 
in  general  features,  which  differ  for  the  most  part  in  intensity  and  in 
frequency  of  occurrence.  The  wide-spread  distribution  of  cyclonic  and  anti- 
cyclonic  disturbances,  their  prevailingly  eastward  movement  around  one 
pole  or  the  other,  the  control  of  weather  changes  by  these  drifting  centers 
of  low  and  high  pressures,  and  the  relation  of  smaller  storms  to  the  larger 
ones,  have  been  completely  established  by  this  vast  fund  of  original  and 
definite  information. 


WEATHER.  329 

Daily  weather  maps  for  the  North  Atlantic  Ocean,  already  referred  to  in 
Sectton  232,  have  been  published  for  the  year  beginning  August,  1882,  by  the 
British  meteorological  council ;  and  for  subsequent  years  jointly  by  the  German 
and  Danish  weather  services.  An  International  Bulletin  was  published  for  a 
number  of  years  by  our  national  Weather  Service,  including  weather  maps  for 
the  entire  northern  hemisphere.  The  data  for  this  extensive  undertaking  were 
secured  from  the  weather  services  of  various  countries,  from  numerous  naval 
and  mercantile  vessels,  and  from  certain  other  observers.  The  chart  of 
eircumpolar  storm  tracks,  Fig.  62,  was  prepared  by  Loomis  from  the  maps 
in  these  Bulletins. 

326.  Weather  proverbs  and  weather  lore.  There  is  a  great  number  of 
popular  sayings  concerning  the  weather  ;  some  being  of  value,  others  deserving 
only  to  be  classed  with  the  superstitions  of  the  middle  ages  concerning  comets 
and  shooting  stars.  A  few  of  these  may  be  considered.  In  our  latitudes,  a 
fine  day  is  often  called  a  weather  breeder,  and  is  said  to  be  too  good  to  last ; 
thus  recognizing  the  generally  rapid  succession  of  bad  weather  after  good.  In 
the  same  way,  it  is  sometimes  pronounced  to  be  too  cold  to  snow  during  cloudy 
weather  in  winter ;  the  cold  resulting  from  a  preceding  anticyclonic  calm  not 
sufficing  to  produce  snow  until  a  southerly  wind  springs  up  in  front  of  the 
next  cyclone  and  raises  the  temperature.  The  increase  of  humidity  in  the 
cooling  southerly  winds  in  front  of  a  cyclone  gives  rise  to  many  prognostics. 
Rheumatic  pains  increase  ;  houses  with  stone  walls,  being  in  summer  somewhat 
cooler  than  the  outer  air,  become  extremely  damp,  the  walls  even  dripping  wet, 
when  the  air  becomes  sultry  before  a  rain  ;  smoke  falls  before  a  storm,  not 
because  the  air  is  then  lighter  from  decrease  of  pressure,  but  because  the 
condensation  of  vapor  on  the  smoke  particles  weighs  them  down.  Dew  formed 
plentifully  after  a  fair  day  and  soon  dissolved  the  next  morning  indicates 
strong  range  of  temperature  under  the  clear  sky  of  an  anticyclone  ;  and  hence 
may  be  taken  to  foretell  a  day  or  two  of  fair  weather,  followed  by  a  cyclonic 
area. 

The  proverbs  relating  to  the  winds  are  very  numerous.  The  immediate 
accompaniments  of  the  winds  are  expressed  when  we  say  a  southerly  wind 
brings  rain ;  a  northwest  wind  brings  cooler  or  colder  weather ;  this  being 
dependent  on  their  position  in  cyclonic  areas.  Of  course,  these  rules  have  to 
be  reversed  in  the  southern  hemisphere.  In  Scotland  it  is  said  "  the  northwest 
wind  is  a  gentleman  and  goes  to  bed  ";  meaning  that  the  nights  are  calm  after 
northwest  wind  by  day  ;  this  follows  naturally  from  the  diurnal  variation  of 
velocity  in  the  clear  weather  of  such  a  wind.  The  changeableness  of  weather 
in  the  temperate  zone  is  expressed  by  the  sailor's  saying  :  "  a  nor'wester  is  not 
long  in  debt  to  a  sou'wester  ";  the  two  being  separated  only  by  the  anticyclonic 
area  or  ridge  of  high  pressure.  The  prevalent  path  of  our  cyclones  is  north  of 


330  ELEMENTARY  METEOROLOGY. 

the  regions  of  greater  population  both  in  America  and  Europe  ;  hence  the 
usual  sequence  of  the  winds  in  clearing  weather  is  from  the  southeast  through 
the  south  to  the  west ;  and  this  is  called  veering  with  the  sun.  If  on  reaching 
the  west  the  wind  backs  through  the  south  towards  the  east,  another  cyclone 
is  coming  with  its  spell  of  bad  weather.  Hence  the  saying  :  "  when  the  wind 
veers  against  the  sun,  trust  it  not,  for  back  it  will  run."  This  saying  is  some- 
times applied  to  the  case  of  storms  that  clear  by  the  backing  of  the  wind  from 
an  easterly  quarter  through  the  north  to  the  northwest,  as  happens  when  the 
storm  passes  south  of  the  observer  :  "  Back  it  will  run  "  is  then  taken  to  mean 
that  another  storm  is  coming ;  but  here  the  saying  does  not  appear  to  have 
good  justification,  for  it  is  plain  that  if  two  equally  competent  observers,  one 
on  the  right  and  the  other  on  the  left  side  of  a  cyclonic  path,  should  employ 
this  rule  to  guide  their  forecasts,  they  would  be  in  continual  contradiction. 
In  northern  Canada,  the  backing  of  the  winds  through  the  north  must  be  the 
ordinary  occurrence.  It  may  be,  however,  when  a  storm  passes  us  to  the 
south,  having  come  along  the  coast  from  the  Gulf  of  Mexico,  and  thus  giving 
backing  winds  as  its  clouds  clear  away,  that  another  cyclone  from  the  north- 
west may  soon  follow,  and  thus  give  some  countenance  to  the  saying. 

Prognostics  from  fog  and  clouds  are  of  much  value.  The  formation  of  fog 
in  valleys  at  night,  and  its  dissipation  early  the  next  morning  indicates  fair 
weather  for  a  time  ;  for  this  implies  clear  anticyclonic  air,  with  active  radia- 
tion at  night  and  warm  sunshine  by  day.  In  the  same  way,  the  dissolution  of 
cumulus  clouds  about  sunset,  only  their  upper  parts  remaining  for  a  time  as 
thin  alto-cumulus  layers,  implies  a  diurnal  control  of  the  weather,  and  hence  a 
fine  morrow,  under  anticyclonic  pressures.  Cirrus  clouds  generally  indicate 
the  approach  of  bad  weather.  "  Mare's  tails  and  mackerel  scales  make  lofty 
ships  carry  low  sails  " ;  these  thin  clouds  being  the  elevated  overflow  of  an 
approaching  cyclone.  At  the  same  time,  the  sun  and  moon  are  paled  by  the 
cirrus  veil ;  they  are .  frequently  surrounded  by  halos  formed  by  refraction  in 
the  ice  crystals  of  such  clouds  ;  the  course  of  the  high  cloud  streaks  is  often 
strongly  different  from  that  of  the  surface  wind  ;  and  all  these  signs  forebode 
a  change  towards  foul  weather.  As  the  clouds  gather  in  lower  levels,  the 
Jight  reflected  at  night  from  iron  furnaces  and  from  the  electric  lights  of 
modern  cities,  is  seen  with  increased  brightness,  and  indicates  the  speedy 
approach  of  rain.  As  the  storm  clouds  pass  by,  a  break  in  the  cloudy  sheet 
showing  enough  clear  blue  sky  "  to  make  a  Scotchman's  jacket,"  or  "  a  Dutch- 
man's breeches,"  as  it  is  variously  expressed,  shows  the  coming  of  fair  weather; 
for  while  breaks  may  often  occur  in  one  cloud  layer  or  another  within  the 
stormy  area,  it  is  very  seldom  that  clear  blue  sky  can  be  seen  through  such 
spaces  ;  but  in  the  rear  of  the  storm,  when  the  lower  clouds  are  gone,  and  the 
high  cirro-stratus  sheet  remains  projecting  backward  from  the  sjborm  center, 
but  drifting  along  with  it,  a  break  discloses  the  bright  blue  sky  above. 


WEATHER.  331 

The  colors  of  the  sky  may  be  often  used  as  prognostics.  A  clear,  fresh 
blue  shows  the  approach  or  presence  of  an  anticyclonic  area,  while  a  pale  sky 
forebodes  an  approaching  cyclone,  even  before  its  cirrus  streamers  appear. 
Halos  are  commonly  formed  around  the  sun  or  moon  in  the  thin  cirro-stratus 
clouds  before  a  cyclone.  A  glaring,  hazy  sky  often  comes  with  southerly 
winds  and  increasingly  hot  weather  in  summer ;  little  dew  is  then  formed  at 
night,  as  radiation  is  checked ;  frosts  need  not  be  expected  at  such  times.  A 
clear  stretch  of  sunset  red  close  along  the  horizon,  surmounted  by  yellows, 
indicates  fair  weather  the  next  day ;  and  especially  so  if  the  rosy  segment  is 
well  displayed  above  the  horizon  colors ;  but  a  lurid  western  sky  at  sunset, 
with  colors  spread  above  the  horizon  on  thin  cirrus  clouds,  indicates  a  coming 
storm  ;  and  if  the  sunset  be  dull  and  "dirty/7  with  clearer  sky  in  the  east,  the 
storm  is  nearer.  Rainbows  in  the  east  and  hence  in  the  afternoon,  foretell 
clearing  weather ;  these  bows  being  generally  formed  on  the  rain  of  retreating 
thunder  showers  (Sect.  290)  ;  but  if  seen  in  the  west  and  therefore  in  the 
morning,  rain  is  approaching.  This  of  course  applies  only  in  those  latitudes 
where  thunder  storms  move  from  west  to  east. 

A  variety  of  sayings  relate  to  the  behavior  of  wild  and  domestic  animals. 
Some  of  these  depend  simply  on  their  behavior  as  affected  by  the  condition  of 
the  weather  at  the  moment,  and  such  may  be  frequently  relied  on  ;  beasts  and 
birds  as  well  as  man  being  disturbed  particularly  by  changes  from  dry  to 
damp  air ;  but  there  is  no  proved  value  in  the  sayings  which  attempt  to  fore- 
tell the  character  of  the  coming  season  by  the  supposed  instinctive  foresight  of 
such  animals  as  bears,  beavers,  moles  and  squirrels,  in  the  preparation  for 
severe  winters  or  long  droughts.  Actual  investigation  has  shown  that  these 
dumb  animals  have  no  such  foresight  as  is  popularly  attributed  to  them. 

No  credence  should  be  attached  to  the  innumerable  sayings  regarding  the 
character  of  certain  seasons  as  determined  by  the  weather  on  certain  dates  of 
the  calendar  ;  a  careful  and  extended  account  over  a  number  of  years  would 
undoubtedly  show  as  many  failures  as  successes  in  such  predictions ;  that  is, 
the  reputation  of  such  weather  proverbs  comes  only  from  "counting  the 
hits  and  forgetting  the  misses,"  as  is  so  common  in  careless  generalizations. 
The  same  comment  may  be  made  regarding  the  days  of  the  week  in  which  the 
phases  of  the  moon  change,  and  the  attitude  of  the  new  moon  in  the  sky.  It  is 
manifest  that  the  tilting  of  the  horns  of  the  new  moon,  for  example,  will 
be  the  same  for  all  observers  on  a  given  latitude  circle ;  and  that  for  a  given 
number  of  days  after  new  moon,  these  observers  will  have  a  great  variety 
of  weather ;  yet  this  lunar  prognostic  would  give  them  all  the  same. 

Nothing  but  a  continued  statistical  study  of  prognostics,  and  a  strict  com- 
parison of  forecasts  with  facts,  will  suffice  to  demonstrate  their  value  ,  yet  no 
such  comparison  has  been  made  by  the  greater  number  of  persons  who  credu- 
lously give  faith  to  oft-repeated  sayings,  believing  that  there  must  be  some- 


332  ELEMENTARY    METEOROLOGY. 

thing  in  them,  because  they  are  often  said.     They  should   be  regarded  as 
survivals  of  superstitious  folk-lore,  rather  than  as  weather-wise  sayings. 

327.  Weather  cycles.  Many  efforts  have  been  made  in  this  country  and 
abroad  to  discover  a  periodic  recurrence  of  weather  changes  of  longer  and  more 
regular  intervals  than  those  between  successive  cyclonic  centers.  A  weekly 
recurrence  of  similar  conditions  has  long  been  known  in  a  general  way,  but 
the  conditions  which  determine  its  duration  and  its  variations  from  perfect 
periodicity  have  not  been  discovered  and  it  has  seldom  been  made  practically 
useful.  It  appears  to  depend  on  a  double  cyclonic  period.  The  control  of  the 
weather  by  the  moon  has  long  been  a  favorite  idea,  but  it  has  not  been  found 
to  bear  the  test  of  accurate  comparisons  of  weather  and  lunar  phases,  except  in 
a  very  faint  and  imperfect  manner.  Whatever  slight  excess  of  one  weather 
element  or  another  there  may  be  at  certain  times  of  the  lunation,  they  have 
no  sufficient  value  for  use  in  weather  prediction.  Thunder  storms  in  Europe 
have  been  found  slightly  more  frequent  at  the  time  of  new  moon  and  first 
quarter ;  but  not  to  a  sufficient  degree  to  warrant  the  use  of  these  poorly 
marked  cycles  in  forecasting.  A  period  corresponding  to  that  of  the  sun's 
rotation,  or  26.7  days,  has  been  found  to  accord  with  a  faint  variation  in  the 
intensity  of  various  weather  elements  ;  and  it  is  argued  that  this  indicates  an 
effect  on  the  atmosphere  produced  by  solar  magnetism  as  well  as  by  solar  heat. 
Besides  these  periods  of  an  astronomical  nature,  there  are  others  of  apparently 
arbitrary  duration,  probably  corresponding  to  the  period  of  the  surges  already 
referred  to  (Sect.  105).  These  have  not  been  sufficiently  studied  to  know  how 
far  they  may  be  useful,  or  to  determine  their  cause  ;  but  they  appear  to  be 
deserving  of  study. 

The  control  of  the  weather  by  the  moon  or  the  planets  still  occasionally 
finds  enough  believers  to  support  the  publication  of  elaborate  long-range 
weather  predictions.  As  these  are  couched  in  general  language  and  intended 
to  be  applicable  to  large  areas  of  the  country,  it  is  not  at  all  difficult  to  gather 
a  number  of  verifications  for  them ;  but  they  are  no  better  than  the  forgotten 
predictions  of  astrology  of  centuries  ago. 


CLIMATE.  333 

CHAPTER   XIV. 

CLIMATE. 

328.  Climate.  The  average  values  of  the  atmospheric  conditions  of  a 
region  constitute  its  climate.  The  most  important  climatic  elements  are  first, 
temperature ;  second,  various  forms  of  moisture,  as  vapor,  cloudiness,  and 
precipitation ;  third,  wind,  including  storms.  The  pressure  of  the  atmosphere 
is  not  a  climatic  element,  and  needs  to  be  considered  in  this  chapter  only  in  its 
association  with  the  divisions  of  the  wind  system. 

While  annual  averages  were  first  considered  in  the  definition  of  climate, 
more  and  more  importance  has  come  to  be  attached  to  the  average  of  seasonal 
values;  and  to  such  special  quantities  as  the  average  highest  or  lowest 
temperature  or  rainfall  of  a  season  or  a  month.  Even  the  extreme  values  are 
often  included  in  climatic  tables,  in  order  to  present  as  fully  as  possible  the 
meteorological  features  of  a  district ;  but  in  so  doing,  we  approach  the  consid- 
eration of  its  weather.  A  full  climatic  account  of  a  locality  should  include  : 
for  temperature  —  the  monthly  and  annual  means,  the  mean  diurnal  range 
for  the  several  months,  the  mean  and  the  absolute  extremes  for  the  year  and 
the  months,  the  mean  diurnal  variability  (the  mean  of  the  differences  between 
the  successive  daily  means),  the  average  dates  of  latest  and  earliest  frost,  the 
average  number  of  days  without  frost ;  the  average  duration  and  value  of 
cyclonic  ranges  of  temperature  in  the  several  months  (Sect.  317);  the  mean 
intensity  of  sunshine  in  clear  weather  of  the  different  months  ;  the  mean 
temperature  of  the  soil  at  successive  depths  down  to  five  or  six  feet :  for 
moisture  —  the  monthly  mean  absolute  and  relative  humidity,  the  mean 
monthly  evaporation  from  a  water  surface;  the  mean  cloudiness  and  mean 
duration  of  sunshine  in  the  several  months ;  the  mean  monthly  and  annual 
rainfall,  with  additional  data  for  melted  snow  in  the  winter  months  ;  the  mean 
number  of  rainy  and  snowy  days  in  every  month,  the  mean  frequency  of  rain- 
fall in  every  month  (number  of  rainy  days  divided  by  the  total  number  of 
days),  the  average  dates  of  latest  and  earliest  snowfall,  the  average  depth 
of  snow  on  the  ground  at  the  end  of  every  month  ;  if  possible,  the  proportion 
of  rainfall  received  from  general  cyclonic  storms  and  from  local  thunder 
storms  in  the  several  months,  and  the  mean  diurnal  variation  of  rainfall  for 
the  different  months  ;  the  mean  number  of  days  with  thunder  storms  and  with 
hail  in  the  several  months  :  for  winds  —  the  frequency  of  different  directions 
for  the  several  months,  with  the  corresponding  mean  velocities,  and  indication 
of  the  frequency  of  calms  and  of  exceptionally  strong  winds  ;  the  mean 
diurnal  variation  in  direction  and  velocity  for  several  months. 


334  ELEMENTARY   METEOROLOGY. 

In  a  region  like  the  eastern  United  States,  the  means  of  climatic  elements 
in  corresponding  months  of  successive  years  vary  so  greatly  that  a  consider- 
able number  of  years  is  required  to  determine  their  true  values.  Hence  the 
importance  of  maintaining  weather  records  continuously  under  conditions  as 
nearly  constant  as  possible,  in  order  to  outlast  the  influence  of  dry  or  wet, 
warm  or  cold  periods.  As  has  already  been  said,  it  is  hardly  worth  while  to 
begin  such  records  unless  there  is  a  fair  probability  of  their  continuance,  and 
unless  good  instruments  can  be  secured  and  properly  exposed. 

329.  Climatic  zones  and  subdivisions.  As  it  is  impossible  to  describe  the 
climate  of  every  locality  in  the  world,  even  if  it  were  known,  it  is  customary 
to  class  together  certain  large  areas  over  which  the  climatic  conditions  are 
similar.  The  earliest  attempt  of  this  kind,  originating  with  the  Greeks, 
considered  only  the  divisions  of  the  earth  as  theoretically  determined  by  the 
geometrical  distribution  of  sunshine  ;  yet  this  suffices  so  well  for  the  purposes 
of  elementary  description  that  it  is  still  followed  in  the  usual  accounts  of  the 
torrid,  temperate,  and  frigid  zones,  whose  definition  by  latitude  circles  has 
already  been  given  in  Chapter  III. 

An  inspection  of  the  isothermal  Charts  I,  II,  and  III,  suffices  to  show  that 
the  regular  boundaries  of  the  zones  are  divergent  from  the  undulating  lines  of 
equal  temperature ;  and  hence  some  authors  propose  to  limit  the  zones  by 
certain  selected  isotherms.  This  plan  has  much  to  recommend  it,  but  it  is 
unsatisfactory  in  giving  undue  prominence  to  temperature  alone,  and  neglecting 
sufficient  consideration  of  other  climatic  factors.  It  is  therefore  here  proposed 
to  consider  climatic  belts  as  limited  by  the  wind  systems  rather  than  by  the 
isotherms  or  the  parallels,  and  thus  secure  a  closer  accordance  with  natural 
atmospheric  areas.  The  limits  thus  determined  do  not  differ  seriously  from  the 
lines  of  latitude ;  they  are  necessarily  somewhat  indefinite,  but  in  this  they 
accord  with  the  gradations  of  nature,  where  sharp  dividing  lines  are  not  drawn. 

There  would  thus  be  recognized  a  torrid  zone,  extending  somewhat  beyond 
the  tropical  circles  to  the  margins  of  the  trade  wind  belts  ;  through  the  middle 
of  this  zone  runs  the  equatorial  belt ;  on  either  margin  lie  disconnected  sub- 
tropical areas.  The  temperate  zones  then  follow  over  the  latitudes  controlled 
by  the  prevailingly  westerly  winds.  They  are  somewhat  narrowed  from  the 
breadth  they  would  have  when  defined  by  the  tropical  circles ;  they  merge  into 
the  frigid  zones,  from  which  the  polar  circles  may  serve  to  separate  them. 
The  temperate  zone  is  not  well  named,  as  its  actual  variations  of  temperature 
are  very  strong  over  large  areas. 

From  what  has  preceded  in  earlier  chapters,  it  is  manifest  that  the  zones 
are  by  no  means  of  uniform  character  over  their  entire  area.  The  north 
temperate  zone  in  particular  includes  areas  so  diverse  climatically  that  no 
simple  description"  can  be  given  of  them  all.  A  subdivision  of  the  /ours 


CLIMATE.  335 

according  to  land  and  water  areas  is  therefore  required ;  and  these  must  again 
be  Subdivided  into  interiors,  western  coasts  and  eastern  coasts.  Finally,  in 
each  zone,  the  striking  differences  determined  by  altitude  above  sea  level  will 
deserve  consideration  under  a  heading  of  mountain  and  plateau  climate.  If 
this  subdivision  be  regarded  as  too  complex,  a  return  may  be  made  to  the 
simpler  division  by  latitude  circles ;  but  it  should  be  remembered  that  effort 
should  be  made  to  recognize  and  classify  natural  features,  rather  than  to  force 
natural  features  into  an  arbitrary  classification.  In  all  cases,  the  general 
characteristics  of  each  zone  and  of  its  subdivisions  should  be  considered,  rather 
than  their  limits,  which  are  necessarily  belts  instead  of  lines. 

330.  The  torrid  zone.     The  margins  of  the   torrid  zone,  as  ordinarily 
defined,  lie  in  latitude  23^°  on  either  side  of  the  equator.     When  defined  by 
isotherms,  they  are  generally  placed  on  the  lines  of  68°  or  70°,  whose  course 
may  be  traced  on  the  chart  for  the  year,  widening  on  the  continents  and 
narrowing  eastward  across  the  oceans  ;  corresponding  nearly  with  the  polar 
limit  of  palms.     When  defined  by  the  polar  margin  of  the  trades,  the  zone 
lies  between  latitudes  30°  and  35°  north  and  south,  widening  somewhat  on 
the  eastern  side  of  the  oceans.     The  area  of  the  zone  according  to  the  latter 
limits  greatly  exceeds  that  of  the  former. 

The  chief  feature  of  the  torrid  zone  according  to  all  these  definitions  is  a 
prevailingly  high  temperature.  The  special  feature  of  the  first  definition  is 
the  small  variation  of  insolation  during  the  year ;  according  to  the  second, 
the  prevailingly  high  mean  annual  temperature ;  according  to  the  third,  a 
comparative  constancy  of  weather  through  the  year,  in  which  the  simplicity 
of  the  climatic  features  is  apparent.  Each  tropical  circle  crosses  the  broad 
trade  wind  belts,  and  thus  separates  these  areas  of  extraordinarily  uniform 
features  into  two  parts :  it  therefore  seems  advisable  to  abandon  these  limits 
and  to  widen  the  torrid  zone  until  it  shall  include  the  whole  belt  of  steady 
winds  and  simple  climate.  The  isothermal  limits  of  the  torrid  zone  narrow 
it  on  the  eastern  side  of  the  oceans,  where  the  action  of  the  winds  tends  to 
maintain  a  constancy  of  seasons,  and  broaden  it  on  the  western  side  of  the 
oceans,  where  a  greater  variation  of  weather  and  a  correspondingly  increased 
complexity  of  climate  is  caused  by  the  extension  of  continental  conditions 
over  a  marine  area ;  it  therefore  again  seems  better  to  adopt  another  division 
whereby  the  equable  and  mild  climates  of  the  eastern  ocean  margins  even  to 
latitude  35°  may  be  associated  with  the  great  uniform  torrid  zone,  and  by 
which  the  more  variable  eastern  coasts  of  China  and  the  United  States  may 
be  associated  with  the  variable  north  temperate  zone. 

331.  The  oceans  of  the  torrid  zone  need  little  additional  description  after 
what  has  been  said  of  their  several  climatic  features  in  the  chapters  on  tern- 


336  ELEMENTARY  METEOROLOGY. 

perature,  winds  and  rainfall;  and  on  the  succession  of  phenomena  in  the 
chapter  on  weather.  The  temperatures  never  reach  extremes,  unless  in  the  lee 
of  a  large  land  area,  as  to  the  west  of  Africa,  where  the  winds  sometimes  carry 
hot  desert  air  in  summer  or  cool  desert  air  in  winter  over  the  sea.  Were  it 
not  for  the  excessive  humidity  by  which  the  discomfort  of  the  equatorial  belt 
is  produced,  the  ocean  area  of  the  torrid  zone  would  deserve  the  name  of 
temperate  better  than  the  northern  zone  to  which  it  is  commonly  applied. 
Its  winds  are  of  unequalled  steadiness  ;  their  interruption  by  cyclones  being 
altogether  exceptional.  Along  its  equatorial  portion,  the  mean  annual  tempera- 
ture is  moderately  increased,  cloudiness  and  rainfall  are  decidedly  increased, 
and  the  winds  are  weakened. 

332.  The  lands  of  the  torrid  zone.  The  torrid  lowlands,  outside  of  the 
sub-equatorial  belts,  are  in  great  part  desert  areas,  or  but  scantily  clothed  with 
vegetation  ;  not  because  of  unfit  temperatures  or  barren  soil,  but  because  of 
deficient  rainfall.  The  Sahara  in  one  hemisphere  and  central  Australia  in 
the  other  give  the  most  extensive  examples  of  the  trade  wind  desert.  On  the 
western  coasts,  the  deserts  come  directly  to  the  sea  shore.  It  is  within  these 
arid  areas  that  the  extremely  high  temperatures  of  the  globe  are  reported. 
With  a  mean  annual  temperature  about  80,  the  mean  of  the  mid-summer  month 
rises  above  90°,  with  extreme  temperatures  of  110°  or  120°.  In  the  mid-winter 
month,  the  temperature  averages  about  70°,  with  minima  seldom  as  low  as 
50°.  The  winds  are  especially  steady  during  the  latter  season  ;  their  constancy 
being  interrupted  chiefly  by  their  regular  nocturnal  cessation,  at  once  a  striking- 
feature  of  both  weather  and  climate  in  torrid  deserts.  The  excessive  damp- 
ness of  the  equatorial  doldrums  is  here  replaced  by  an  excessive  dustiness 
of  the  air. 

The  rainy  equatorial  belt  on  land  determines  the  extension  of  the  great 
equatorial  forests  of  middle  Africa  and  of  the  Amazon.  Here  the  heat  of  tin? 
hot  season  is  not  so  excessive  as  on  the  dry  deserts  some  distance  from  the 
equator  ;  here  is  the  luxuriance  of  life  which  we  commonly  associate  with  too 
large  a  part  of  the  torrid  zone.  Fortunately  for  the  world,  the  breadth  of  the 
rainy  belt  on  the  lands  is  increased  over  its  narrower  limits  on  the  ocean,  and 
thus  the  area  of  the  adjoining  trade  wind  deserts  is  encroached  upon.  As  the 
rainfall  of  the  sub-equatorial  belts  decreases,  so  the  vegetation  diminishes  ; 
dense  jungles  of  the  equator  give  way  to  more  open  forests  ;  and  these  in 
turn  to  a  region  of  scattered  trees  with  intervening  grassy  spaces  ;  further  on, 
the  trees  disappear  except  along  the  rivers,  the  grasses  become  sparse  and 
patchy ;  then  only  a  desert  flora  can  survive  the  long  dry  season,  and  finally 
even  this  is  lost,  leaving  the  ground  barren. 

The  Indian  monsoon  region  has  a  climate  of  its  own.  Here  the  division  of 
the  year  into  three  seasons  is  the  most  marked  feature  ;  a  cold  season,  when 


CLIMATE.  337 

the  normal  trade  wind  holds  sway,  with  little  precipitation  except  in  the  more 
northern  part  of  the  peninsula ;  a  hot  season,  when  the  northward  march 
of  the  sun  raises  the  temperature  to  an  excessive  degree ;  a  wet  season,  when 
the  winds  weaken  and  change,  and  the  moist  southerly  monsoon  blows  from 
the  sea  over  the  land  with  heavy  clouds  and  rain.  This  is  like  the  Saharan 
climate,  with  the  addition  of  the  equatorial  climate  for  a  part  of  the  year. 
Northwestern  India,  known  as  the  Punjab,  or  Five-river  district,  manifests 
this  succession  of  seasons  with  much  distinctness,  as  appears  from  the  follow- 
ing abstract  from  an  account  by  a  resident :  From  April  to  June,  there  is 
little  or  no  rain ;  the  west  wind  blows  from  the  deserts  of  the  lower  Indus 
with  a  desicating,  scorching  heat,  as  if  from  a  furnace.  The  thermometer  in 
the  shade  may  rise  above  120°.  The  nights  are  relatively  cool,  and  then  the 
houses  are  opened  for  ventilation.  It  is  only  in  the  early  morning  that  an 
enjoyable  air  is  found  for  exercise.  During  the  day-time,  the  houses  should  be 
kept  closed.  Vegetation  withers  ;  the  grass  seems  burnt  to  the  roots  ;  the 
ground  is  hard  and  cracked.  Before  the  wet  season  opens,  the  winds  weaken 
and  the  heat  seems  even  more  severe  than  before.  In  the  later  summer,  the 
rains  come  as  cyclonic  storms  of  increasing  intensity,  yielding  a  plentiful 
rainfall  over  much  of  the  country.  Trees  burst  into  leaf  a  second  time ;  the 
grass  springs  up  and  a  vegetation  is  soon  developed  that  can  be  scarcely  kept 
within  bounds.  The  breaks  in  the  rains  are  oppressively  hot  and  sultry  ;  and 
for  a  time  after  the  rainy  season  closes,  the  heat  and  dampness  are  excessive, 
making  September  the  most  unhealthy  month  of  the  year.  In  October,  the 
northerly  winds  set  in  steadily,  clearing  the  skies  ;  the  temperature  is  then 
pleasant,  and  by  the  end  of  the  year  the  nights  become  cold.  Rain  falls 
in  moderate  amount  from  cyclonic  storms  which  move  eastward,  bringing  cold 
northerly  winds  after  them,  as  if  they  belonged  to  the  procession  of  storms  in 
the  temperate  zone ;  thus  suggesting  that  northern  India,  which  is  so  truly 
tropical  in  its  hot  and  wet  season,  partakes  of  the  features  of  the  temperate 
zone  in  winter.  In  February,  there  is  a  short  spring,  tempting  a  growth  of 
vegetation  ;  but  this  is  followed  by  a  rapid  increase  of  temperature,  and  the 
hot  season  is  again  at  hand. 

Two  subordinate  divisions  of  the  torrid  zone  remain  to  be  mentioned  :  the 
literal  and  the  mountain  climates.  The  torrid  coasts  are  generally  well- 
watered,  and  their  diurnal  temperature  is  modified  by  the  sea  breeze.  Western 
coasts  in  trade  wind  latitudes  form  exceptions  to  this  rule,  as  they  are  pre- 
vailingly barren ;  but  from  this  exception  there  is  again  a  departure  in  the 
Indian  monsoon  region,  where  the  western  coasts  are  better  watered  than 
the  eastern  coasts. 

The  mountain  climate  of  the  torrid  zone  is  tempered  by  altitude.  In 
equatorial  Africa,  mountain  peaks  ascend  even  high  enough  to  hold  snow  the 
year  round  ;  in  rising  from  the  torrid  forests  around  the  mountain  base,  one 


338  ELEMENTARY    METEOROLOGY. 

may  pass  through  successive  zones  of  vegetation  until  the  cold  of  the  upper  air 
prevents  all  plant  life.  In  Ceylon,  the  highest  mountain  bears  certain  plants 
like  those  of  England,  while  the  lowlands  exhibit  the  greatest  tropical  luxuri- 
ance. On  the  plateau  of  southern  India,  6,000  or  7,000  feet  above  sea  level, 
the  days  are  warm  and  the  nights  cool,  but  the  mean  temperature  is  moderate 
all  the  year  round ;  even  in  the  wet  season  the  rainfall  is  not  excessive,  as  the 
higher  hills  to  the  west  receive  the  greater  part  of  it.  "  Many  an  old  Anglo. 
Indian,  whom  choice  or  necessity  has  led  to  fix  his  home  in  India,  has  found  in 
these  hills  scenery  as  beautiful  and  a  climate  as  enjoyable  as  any  in  the  most 
favoured  lands  of  the  Mediterranean  shores."  The  rainfall  of  the  highlands 
also  departs  from  the  rule  of  the  lowlands ;  for  mountains  often  provoke  pre- 
%cipitation  that  might  not  otherwise  occur.  Thus  the  highlands  of  Brazil 
cause  the  growth  of  many  a  cloud  from  which  the  rain-trail  drifts  across  the 
adjacent  valleys ;  in  the  absence  of  the  mountains,  the  dryness  of  the. interior 
plains  would  extend  closer  to  the  coast.  Even  the  barren  desert  of  the  Sahara 
contains  mountains  that  cause  sufficient  rainfall  to  support  forests,  and  yield 
streams  that  wither  as  they  descend  to  the  lower  country.  The  bountiful 
rainfall  of  the  Malayan  islands  is  due  chiefly  to  their  combination  of  equa- 
torial, literal  and  mountainous  conditions.  Their  accessibility  and  their  pro- 
ductiveness destine  them  to  play  an  important  part  in  the  higher  development 
of  the  world. 

333,  The  transitional  sub-tropical  areas  between  the  torrid  and  temperate 
zones  are  of  much  greater  importance  in  certain  longitudes  than  in  others. 
On  the  lands  around  the  Mediterranean,  on  the  middle  western  coasts  of  North 
and  South  America,  along  the  southern  coast  of  Australia,  and  in  South 
Africa,  their  characteristic  features  are  distinctly  brought  out  in  the  dryness 
and  warmth  of  the  nearly  continuous  fair  weather  of  the  summers,  and  in  the 
cloudiness  and  fair  supply  of  rainfall  that  come  with  the  more  stormy  weather 
of  the  winters.  These  areas,  at  one  season  dominated  by  the  inflow  at  the 
source  of  the  trade  winds,  at  another  by  the  stormy  winds  of  the  temperate 
zone,  alternately  associate  themselves  with  the  zones  that  they  adjoin.  They 
are  far  enough  from  the  equator  to  avoid  the  excessive  heats  of  the  torrid 
zone ;  and  their  situation  with  respect  to  land  and  sea  prevents  their  invasion 
by  the  low  temperatures  of  higher  latitudes. 

The  climate  of  these  ar,eas  is  among  the  most  delightful  of  the  world.  The 
attractions  of  the  health  resorts  of  the  Mediterranean  have  long  been  well 
known.  The  equally  delightful  climate  of  southern  California  has  in  recent 
years  gained  a  deserved  appreciation  in  this  country.  Although  in  the  same 
latitude  as  our  middle  Atlantic  coast,  it  has  none  of  our  hot  or  cold  waves ;  its 
skies  are  prevailingly  clear,  and  even  in  its  winter  wet  season,  its  rains  are 
light.  In  regularity  of  succession  of  diurnal  features,  and  in  constancy  of 


CLIMATE.  339 

in* -an  temperatures  through  the  year,  it  rivals  the  greater  part  of  the  torrid 
lands. 

It  does  not  appear  advisable  to  continue  the  sub-tropical  areas  around 
the  world.  On  the  oceans,  they  may  follow  the  belt  of  high  pressures,  in 
which  the  winds  are  weak,  the  skies  prevailingly  clear,  and  the  air  fresh.  But 
the  continental  interiors  on  these  latitudes  possess  seasonal  variations  so 
strong  that  they  should  be  associated  with  the  temperate  zone ;  and  the  eastern 
coasts,  even  nearer  the  equator  than  the  sub-tropical  areas  of  the  western 
coasts,  are  more  naturally  associated  with  the  land  areas  on  their  polar  and 
western  sides.  Their  annual  range  of  temperature  exceeds  that  of  the 
tempered  western  coasts ;  their  rainfall  occurs  in  summer  rather  than  in 
winter,  like  that  of  the  interiors.  Thus  Florida,  lying  five  degrees  south 
of  southern  California,  will  be  grouped  with  the  temperate  zone,  which  widens 
eastward  in  crossing  the  continents,  rather  than  with  the  sub-tropical  areas, 
which  widen  in  crossing  the  seas.  Southern  China,  near  the  middle  latitude 
of  the  Sahara,  will  be  grouped  in  the  same  way. 

334,  The  south  temperate  zone.  The  temperate  zones  of  the  two 
hemispheres  are  so  unlike  that  they  must  be  described  separately.  The  great 
water  zone  of  the  southern  hemisphere  will  sufficiently  represent  the  smaller 
northern  ocean  areas;  the  broad  northern  continents  will  exaggerate  the 
climatic  features  of  the  restricted  southern  islands.  The  southern  temperate 
zone,  lying  beyond  the  axis  of  the  tropical  high  pressure  belt,  is  chiefly 
an  ocean  zone.  It  is  characterized  by  prevailingly  stormy  westerly  winds, 
prevailingly  low  temperature,  and  frequent  cloudiness  and  precipitation, 
rather  than  by  the  seasonal  variation  of  these  elements.  Owing  to  the  absence 
of  land  barriers,  the  southern  ocean  currents  wheel  round  and  round  their 
polar  center,  with  few  pronounced  north  or  south  deflections,  such  as  occur  in 
the  corresponding  latitudes  of  our  hemisphere  ;  hence  the  southern  temperate 
zone  is  of  extraordinarily  uniform  features  all  around  its  circuit.  In  spite  of 
the  strong  variation  of  insolation  with  the  southing  and  northing  of  the  sun, 
the  range  of  mean  monthly  temperatures  from  January  to  July  is  hardly 
greater  than  at  many  parts  of  the  torrid  oceans  ;  so  effective  is  the  conservative 
action  of  the  ocean  waters,  and  of  the  fogs  and  clouds  by  which  they  are  so 
generally  shielded ;  but  winter  is  the  season  of  stronger  winds  and  heavier 
precipitation.  In  those  more  detailed  climatic  elements,  which  take  account 
of  the  monthly  and  inter-diurnal  ranges  of  temperature,  there  is  a  great 
difference  between  the  torrid  and  the  south  temperate  zones,  because  the 
former  is  controlled  so  largely  by  diurnal  processes,  and  the  latter  is  so 
completely  in  the  hands  of  cyclonic  processes,  as  has  been  stated  in  the 
chapter  on  weather.  The  few  land  areas  that  interrupt  the  south  temperate 
ocean  zone  are  most  inhospitable ;  not  that  their  winters  are  exceptionally 


340  ELEMENTARY    METEOROLOGY. 

cold,  for  they  know  nothing  of  the  extremely -low  temperatures  of  northern 
continental  interiors  of  corresponding,  or  even  of  lower  latitudes ;  but  that 
their  chilling  summers  are  so  little  warmer  than  their  winters.  The  tempera- 
ture finds  its  summer  maxima  about  40°  and  50°.  Snows  are  not  uncommon 
even  in  January  or  February,  when  the  weather  should  be  mildest.  There  is 
therefore  no  mild  season  in  which  provision  may  be  made  for  the  remainder  of 
the  year,  as  there  might  be  if  the  lands  were  larger.  When  the  uninhabited 
island  of  South  Georgia,  with  glaciers  descending  into  the  sea,  is  compared 
with  middle  England,  to  which  it  corresponds  in  latitude,  the  contrast  is 
remarkable  ;  but  this  is  more  because  of  the  exceptionally  favorable  condition 
of  England  in  the  north  temperate  zone  than  of  the  peculiar  quality  of  the 
South  Georgian  climate  in  the  south  temperate  zone. 

335-  The  north  temperate  zone  is  the  great  land  zone  of  the  world.  It  is 
of  particular  importance  as  the  chief  seat  of  modern  civilization.  Its  oceans 
need  little  consideration,  as  they  repeat  the  features  of  the  south  temperate 
zone,  modified  by  an  approach  to  the  land  climate  on  the  western  side  of  the 
oceans,  and  by  the  diverse  courses  of  the  ocean  currents,  especially  on  the 
eastern  side  of  the  oceans.  On  the  broad  northern  lands,  the  variation  of  the 
seasons  and  the  inconstancy  of  the  weather  become  the  dominating  climatic 
features ;  and  annual  averages,  which  almost  suffice  to  define  the  climate  of  the 
torrid  oceans,  are  very  misleading.  The  actual  temperatures  of  the  year  reside 
longer  near  the  temperatures  of  the  hottest  or  coldest  month  than  near  the 
annual  mean ;  hence  the  hot  and  cold  seasons  are  strongly  separated  by 
relatively  short  warming  and  cooling  seasons.  The  interior  of  the  lands  will 
be  first  considered.  Western  Europe  and  the  eastern  United  States,  standing 
in  different  relations  to  their  continental  centers,  will  be  then  taken  as  types 
of  leeward  and  windward  coastal  areas. 

The  southern  portion  of  the  continental  interiors  attain  truly  torrid  heats 
in  their  summer  season  ;  Arizona  and  Persia  rival  the  Sahara  in  having  mean 
temperatures  for  July  above  90°.  Their  lowlands  are  deserts ;  their  scanty 
rains  come  chiefiy  from  showers  that  begin  on  the  neighboring  mountains,  or 
from  violent  thunder  storms  that  deliver  a  whole  season's  precipitation  in  a 
few  hours.  Their  winters  are  cool,  with  a  January  mean  of  about  50°,  and 
minimum  often  down  to  freezing.  The  northern  part  of  the  interiors  have 
warm  summers,  with  a  July  mean  of  about  60°;  but  toward  the  Arctic  border 
they  are  extremely  cold  in  the  winter  season,  when  the  January  mean  over 
large  districts  is  as  low  as  — 20°;  while  in  a  limited  district  in  northeastern 
Siberia  it  sinks  below  —  40°  or  —  50° ;  and  in  the  associated  margin  of  the 
frigid  zone,  at  the  town  of  Verkoyansk,  even  to  —  60°.  In  these  hard  frozen 
regions,  the  underground  temperature  remains  below  the  freezing  point  the 
year  round,  trees  are  stunted  or  absent,  and  crops  grow  only  in  the  surface 


CLIMATE.  341 

layer  of  soil  that  melts  under  the  sunshine  of  long  summer  days.  Over  the 
greater  part  of  these  extended  land  areas,  the  rainfall  has  a  more  or  less 
distinct  maximum  in  summer;  in  the  far  interiors,  the  winter  snowfall  is 
moderate  in  spite  of  the  severe  cold. 

A  large  interior  area  of  North  America,  stretching  from  Missouri  northward 
past  Winnipeg  and  eastward  beyond  the  Great  Lakes,  possesses  in  a  marked 
degree  the  variable  features  of  this  so-called  temperate  climate.  The  summers 
are  warm,  with  spells  of  extreme  heat  often  broken  by  destructive  local  storms 
from  which  the  greater  part  of  the  rainfall  is  obtained ;  the  winters  are  cold, 
with  times  of  excessively  low  temperature,  brought  by  stormy  winds  that  cause 
rapid  changes  from  one  extreme  to  another ;  vast  floods  of  freezing  air  from 
the  north  are  the  scourge  of  the  winter,  as  the  violent  local  storms  are  of  the 
summer.  Land  regions  with  a  climate  such  as  this  have  little  association  with 
the  south  temperate  ocean  zone  and  its  comparatively  equable  and  regular 
though  rough  climatic  features.  The  northern  land  interiors  can  hardly  be 
classed  with  the  narrower  oceans  between  them.  In  the  presence  of  these 
regions  of  extremes,  separated  by  oceans  of  relatively  equable  climate,  and 
unrepresented  in  the  southern  hemisphere,  the  effort  to  divide  the  earth 
into  symmetrical  climatic  zones  bounded  by  latitude  lines  can  hardly  be 
accomplished. 

336.  The  coasts  of  the  north  temperate  zone.  The  extreme,  continental 
or  interior  climates,  as  they  are  variously  called,  of  the  interior  lands  in  the 
north  temperate  zone  are  strongly  contrasted  with  the  relatively  equable 
climate  of  the  western  coastal  lands  in  middle  latitudes,  particularly  with 
that  of  the  favored  western  coast  of  Europe,  as  illustrated  in  Fig.  18.  There, 
on  the  same  latitude  with  interiors  having  a  January  mean  temperature  of 
—10°  or  —20°,  the  coast  of  France  and  the  British  Isles  enjoy  a  mean  of  40° 
or  50°;  and  in  summer  when  the  interiors  have  July  means  of  70°  or  80°,  the 
favored  coast  rises  only  to  60°  or  70°.  Like  the  temperate  oceans  which  they 
adjoin,  their  winters  are  wetter  than  their  summers ;  but  they  do  not  suffer 
from  periodical  droughts,  like  the  sub-tropical  western  coasts  nearer  the  equator. 
The  air  is  damp,  with  much  cloudiness,  especially  in  winter.  In  moderation 
of  mean  annual  temperature  range,  the  western  coast  of  Europe,  and  to  a 
somewhat  less  degree  the  western  coast  of  North  America,  vie  with  a  great 
part  of  the  torrid  zone ;  although  the  daily  temperatures  from  which  the 
monthly  means  are  computed  exhibit  variations  much  greater  on  temperate 
coasts  than  in  torrid  latitudes,  on  account  of  the  cyclonic  fluctuations  of 
the  westerly  winds.  Inland  from  the  open  and  irregular  coast  of  western 
Europe,  this  mild  climate  gradually  merges  across  the  lowlands  into  the  more 
severe  climate  of  the  interior  ;  the  temperatures  vary  over  a  greater  range  ;  the 
rainfall  for  a  time  is  nearly  equably  distributed  over  the  year,  and  then  takes 


342  ELEMENTARY   METEOROLOGY. 

cm  a  summer  maximum  ;  the  frequent  cloudiness  and  high  humidity  of  the 
coast  is  exchanged  for  clearer  skies  and  drier  air.  A  similar  change  is  found 
in  passing  eastward  from  our  Pacific  coast,  but  here  it  is  made  abruptly.  On 
crossing  the  lofty  mountains  which  here  lie  so  little  distance  inland,  we  enter 
at  once  on  the  extreme  climate  of  the  interior  ;  the  torrid  heat  of  Arizona 
and  the  extreme  cold  of  the  Mackenzie  basin  with  its  excessive  annual  range 
lying  only  a  few  hundred  miles  from  the  mild  litoral  belt  by  the  ocean. 

The  eastern  coasts  of  the  temperate  continents,  represented  by  our  Atlantic 
seaboard  and  that  of  eastern  Canada,  might  be  classified  with  the  interiors, 
from  which  they  derive  so  many  of  their  climatic  features  ;  but  for  the  purpose 
of  emphasizing  their  contrasts  with  the  western  coasts,  they  are  here  given 
a  special  paragraph.  They  partake  of  the  strong  temperature  ranges,  both 
annual  and  cyclonic,  that  characterize  the  interiors,  because  the  general  drift 
of  the  winds  is  here  from  land  to  sea.  Not  only  so;  the  seasonal  shifts  of 
the  winds  (Sect.  155)  here  intensify  the  seasonal  variations  of  temperature  ; 
in  summer,  the  prevailing  winds  are  from  the  over-warm  southwest  lands  ;  in 
winter,  from  the  over-cold  northwest  lands.  In  this,  they  are  reversed  from 
the  relations  obtaining  on  the  western  coasts  ;  the  summer  winds  there  come 
prevailingly  from  the  little-warmed  northern  seas  ;  and  the  winter  winds  from 
the  little-cooled  seas  of  lower  latitudes.  The  rainfall  is  generally  well  dis- 
tributed through  the  year  along  our  eastern  coast ;  but  on  the  corresponding 
coast  of  China,  the  winters  are  relatively  dry,  and  the  rains  of  summer  are  in 
the  control  of  the  southerly  monsoon. 

One  of  the  distinct  peculiarities  of  the  climate  of  our  eastern  temperate 
coasts  is  its  rapid  increase  of  severity  poleward.  In  spite  of  a  strong  range 
of  temperature,  our  southern  Atlantic  coast  must  be  included  among  the  better 
favored  regions  of  the  world  ;  but  in  passing  northward,  over  a  latitude  range  no 
more  than  that  from  Morocco  to  Scotland,  we  pass  from  Carolina  to  Labrador ; 
and  there,  opposite  to  the  mild  climate  of  Great  Britain,  we  find  inhospitable 
shores  around  a  forbidding  interior ;  England,  a  land  of  mild  and  genial 
climate,  in  which  so  small  a  river  as  the  Thames  is  only  exceptionally  frozen 
over ;  Labrador,  an  almost  uninhabitable  region,  with  a  January  mean  but 
little  over  zero,  swept  over  by  harsh  winds  from  a  vast  snow-covered  interior. 
When  this  striking  contrast  was  first  recognized  two  hundred  years  ago,  it  so 
strongly  impressed  Halley,  the  eminent  English  astronomer  of  that  time,  that 
lie  thought  the  <-<>l<l  of  nort  hnistern  America  resulted  from  the  North  Pole 
once  having  occupied  that  part  of  the  earth's  surface. 

The  contrasts  thus  presented  on  an  east  and  west  line  in  middle  temperate 
latitudes  deserve  especial  attention,  and  again  serve  to  illustrate  the  difficulty 
of  fitting  any  simple  system  of  climatic  zones  to  the  complications  of  nature. 
The  latitude  circle  of  50°  N,  in  summer  or  winter,  traverses  regions  so  dissimilar 
that  to  include  them  all  under  a  single  zone  would  defeat  the  object  of  this 


CLIMATE.  343 

chapter.  Beginning  on  the  equable  Atlantic  waters,  the  circle  enters  middle 
Europe  where  the  history  of  the  last  thousand  years  has  witnessed  the  greatest 
progress  that  the  world  has  ever  seen.  It  crosses  the  broad  deserts  of  central 
Asia,  where  the  monotonous  extent  of  too  much  land,  too  far  from  the  vapor- 
yielding  seas,  degrades  its  inhabitants,  and  holds  them  close  down  to  barbarism. 
Emerging  on  the  Pacific  coast,  it  finds  the  more  populous  countries  to  the 
south,  its  own  climate  being  too  harsh  for  easy  occupation.  After  crossing 
the  broad  and  equable  northern  Pacific,  it  traverses  the  narrow  belt  of  tempered 
and  moist  climate  in  British  Columbia,  and  then  beyond  a  rugged  mountain 
region,  with  many  snowy  peaks,  it  discovers  the  severe  interior  climate  of  the 
Saskatchewan,  and  the  sparsely  settled  district  between  our  Great  Lakes  and 
Hudson  Bay.  On  the  desolate  Labrador  coast,  the  better  favored  climate 
and  populous  regions  are  again  found  to  lie  further  to  the  south.  As  far  as 
habitability  is  concerned,  this  middle  temperate  belt  contains  climatic  variations 
almost  as  great  as  are  encountered  in  passing  from  the  equator  to  the  pole. 

337.  The  mountain  climate  of  the  temperate  zone  is  marked  by  an 
increased  intensity  of  insolation,  a  decrease  of  temperature  and  an  increase, 
up  to  certain  altitudes,  of  precipitation  similar  to  that  already  described  for 
the  torrid  zone.  Ascending  from  the  dry  sub-tropical  lowlands  of  southern 
California,  one  soon  leaves  the  orange  groves,  passes  up  the  slopes  of  the 
Sierra  Nevada  through  forests  at  first  deciduous,  but  in  which  coniferous  trees 
rapidly  increase  in  numbers,  and  at  last  above  the  tree-line,  snow-fields  are 
found  on  the  higher  peaks  that  last  through  the  year  from  the  heavy  fall  of 
winter.  The  lofty  plateaus  of  Arizona  and  southern  Utah  rise  above  desert 
valleys  and  bear  abundant  forests,  supported  by  a  sufficient  rainfall ;  one  of 
the  plateaus  is  well  named  the  Aquarius.  The  volcanic  summit  of  San  Fran- 
cisco mountain  in  Arizona  rises  to  an  elevation  of  12,800  feet,  a  mile  above 
the  desert  table  land  around  its  foot.  Well-marked  zones  of  vegetation, 
including  a  belt  of  heavy  forest,  have  been  recognized  on  its  conical  slopes ; 
they  stand  obliquely,  a  little  lower  on  the  shady  northeast  than  on  the  sunny 
southwest  side.  The  uppermost  of  these  zones,  above  the  tree  line,  contains  a 
number  of  Arctic  plants,  nine  of  which  are  identical  with  plants  brought  by 
General  Greely  from  Lady  Franklin  Bay,  near  latitude  82°  N. 

Besides  these  features  of  reduced  temperature  and  increased  precipitation, 
there  is  an  increased  windiness  with  a  nocturnal  maximum,  and  during  clear 
weather  an  active  evaporation,  which  in  part  compensated  for  the  frequently 
increased  cloudiness.  The  mean  diurnal  and  annual  ranges  of  temperature  are 
less  than  on  adjacent  interior  lowlands.  Lofty  plateaus  differ  from  mountain 
peaks  of  similar  altitude,  by  a  relatively  higher  temperature,  an  increased 
range  of  temperature  and  a  distinct  diurnal  wind  period,  with  maximum  in 
the  afternoon. 


344  ELEMENTARY    METEOROLOGY. 

338.  The  frigid  zones.     The  leading  characteristic  of  the  frigid  zones  in 
the  Arctic  and  Antarctic  regions  is  a  persistently  low  temperature,  in  spite  of 
great  variations  of  insolation,  winter  and  summer.     It  is  true  that  during  the 
long  night  of  winter,  the  temperature  falls  to  extremely  low  degrees  ;  and  that 
under  the    ong  days  of  summer,  it  rises  much  above  its  winter  mean ;  but  at  no 
time  of  yd   r  is  there  what  may  properly  be  called  a  warm  season.     The  tem- 
perature i:      "Idom  much  above  freezing ;    snow  falls  frequently  in  summer, 
although  £        alternating  with  rain ;  the  summer  is  therefore  only  a  less  pro- 
nounced wir    T,  as  far  as  fitness  for  occupation  is  concerned.     Both  animals 
and  plants       e  greatly  reduced  in  variety  and  number  on  the  lands,  but  the 
polar  seas  h  ve  a  considerable  fauna  and  flora.     On  account  of  the  prevalence 
of  snow  fields  and  glaciers  upon  the  lands,  an  exaggerated  estimate  of  the 
annual  precipitation  is  prevalent.     It  is  in  fact  moderate ;  behig  generally  less 
than  fifteen  inches,  and  at  certain  places  under  ten  or  even  under  five  inches. 
Owing  to  tin  low  temperature  of  the  air,  evaporation  is   slow;    hence  the 
snow  suffers  little  either  by  melting  or  by  passing  off  as  vapor  into  the  air, 
but  is  economized  and  thus  accumulates  over  large  areas,  forming  glaciers  and 
ice  sheets  descending  to  or  near  the  sea ;  the  greatest  of  these  being  the  ice 
field  of  Greenland,  by  which  the  greater  part  of  that  land  is  concealed.  '  The 
weakness  of  the  diurnal  element  in  the  polar  climate  is  a  natural  sequence  of 
its  weakness    in  weather  changes,  as  already  described.      Spells  of   stormy 
weather,  unbroken  by  changes  from  day  to  night,  constitute  a  marked  feature 
of  the  frigid  zones.    The  winds  are  variable  ;  but  come  not  infrequently  from  a 
northern  quarter  in  the  Arctic  zone,  as  has  been  explained  in  Section  158  ;  in 
the  Antarctic  zone,  they  are  more  prevalent  from  the  westerly  quarter,  as  far 
as  known.     In  summer,  calm  spells  are  relatively  warm ;  but  in  winter  they 
are  the  times  of  greatest  cold.     Wind  from  any  direction  then  causes  a  rise  of 
temperature,  probably  because  the  cold  of   winter  calms  is  caused  by  local 
radiation  ;  the  effect  of  which  is  lessened  when  winds  mix  the  lower  and  upper 
layers  of  the  atmosphere.      The  absolute  humidity  is  of  course  low,  as  but 
little  vapor  can  exist  at  temperatures  here  prevailing ;  the  relative  humidity 
varies  greatly;  when  high,  the  feeling  of  cold  is  greatly  increased;  when  low, 
the  cold  is  much  more  easily  borne.     Arctic  travellers  complain  of  suffering 
from  thirst ;  probably  because  of  evaporation  from  the  surface  of  the  body, 
whose  temperature  is  necessarily  much  higher  than  that  of  the  air. 

339.  Climatic  control  of  habitability.     The  conditions  of  occupation  of 
the  earth,  and  especially  of  the  lands,  by  plants,  animals  and  man  depend  in 
much  greater  degree  on  climate  than  on  any  other  factors.     Over  limited 
areas  and  for  a  limited  time,  a  fresh  lava  flow  may  remain  barren  until  a  soil 
is  gradually  formed  over  it ;  but  all  the  other  deserts  of  the  world  are  climatic. 
The  snowy  deserts  of  the  polar  regions  and  of  high  mountains  depend  on  the 


CLIMATE.  346 

prevailing  low  temperature ;  the  sandy  and  stony  deserts  of  trade  wind  belts, 
of  areas  in  the  lee  of  lofty  mountain  ranges  and  of  interior  basins,  depend  on 
the  prevailing  low  humidity.  The  occasional  salt  deserts  of  the  world  also 
belong  in  the  latter  class.  In  milder  and  moister  climates,  all  these  areas 
might  support  a  plentiful  fauna  and  flora. 

The  first  essential  condition  for  the  support  of  land  plants  and:after  them 
of  animals,  is  the  preparation  of  a  covering  of  soil,  formed  by  J '  ^disintegra- 
tion of  the  underlying  rock  ;  and  there  are  no  known  rocks  ti  'Will  not  in 
a  sufficiently  long  time  weather  into  a  loose  plant-supporting  '  ft.  Locally, 
the  loosened  material  may  be  carried  away  as  fast  as  it  is  formee'.''  as  from  the 
rocky  slopes  of  mountains  and  from  the  ledges  of  hills  ;  but  this  unexceptional. 
The  second  condition  is  a  fitting  climate.  In  too  dry  a  region,  the  soil  may 
be  produced,  but  it  lies  sterile  ;  or  the  finer  parts  may  be  blown  away,  leaving 
a  sandy  or  stony  surface,  nearly  or  quite  barren.  In  too  cold  a  climate,  the 
ground  is  frozen,  snow  accumulates  over  it,  and  nearly  all  life1  is  excluded. 
But  where  the  rainfall  is  sufficient,  —  that  is,  over  10  or  15  inches  a  year,  — 
and  where  the  cold  is  not  excessive,  so  that  during  at  least  a  part  of  the  year 
the  ground  is  melted,  and  the  temperature  is  sufficiently  high  to  encourage 
flowering  and  fruiting,  plants  and  animals  may  survive,  their  forms  and  habits 
being  specially  adapted  to  the  unfavorable  conditions  in  which  they  live. 

From  these  limiting  conditions,  an  increasing  variety  of  life  is  found  with 
increasing  warmth  and  humidity,  until  the  luxuriance  of  the  sub-equatorial 
belt  is  reached.  It  is  noteworthy  that  a  moderately  warm  summer  alternating 
with  a  severe  winter,  such  as  occurs  in  the  northern  continental  interiors,  is 
much  more  favorable  for  the  development  of  varied  forms  of  life  than  the 
equable  but  inhospitable  climate  of  the  remote  islands  in  the  boundless  seas 
of  the  south  temperate  zone.  It  is  also  instructive  to  see  that  the  climatic 
conditions  most  favorable  for  man  are  not  those  of  the  equatorial  lands,  where 
the  temperature  is  enervating  and  where  the  support  of  life  presents  no  diffi- 
culty and  calls  for  little  forethought;  but  those  of  the  northern  temperate 
zone,  where  the  rigors  of  the  coming  winter  season  call  for  a  thoughtful 
preparation  during  the  preceding  summer. 

340.  Local  control  of  climate.  In  the  chapter  on  rainfall,  mention  was 
made  of  the  attempts  to  produce  artificial  rain;  either  temporarily,  as  by 
extensive  fires  or  by  explosions,  or  permanently,  by  planting  trees.  Observa- 
tions thus  far  made  do  not  give  encouragement  to  these  projects.  On  the 
other  hand,  it  has  been  proved  that  areas  of  forest  amid  an  open  country  are 
in  a  small  way  conservative  in  their  influence,  decreasing  the  suddenness  of 
the  changes  that  take  place  about  them.  .  The  rain  that  falls  does  not  run 
away  so  quickly,  and  therefore  not  only  provides  a  better  supply  for  the  steady 
running  of  streams,  but  also  decreases  the  loss  of  soil  by  surface  washing. 


346  ELEMENTARY  METEOROLOGY. 

The  variations  of  temperature  in  the  forest  air  and  in  the  soil  beneath  are 
less  than  in  the  surrounding  district.  In  mountainous  regions,  the  presence 
of  forests  is  important  in  restraining  floods  and  in  holding  the  soil  on  the 
slopes  ;  but  in  our  western  semi-arid  plains,  it  is  doubtful  if  even  these 
indirect  climatic  conditions  will  be  seriously  affected  by  any  possible  tree 
planting;  much  less  will  the  amount  of  rainfall  be  changed.  The  more  general 
movements  of  the  lower  atmosphere,  on  which  temperature  and  rainfall  so 
largely  depend,  are  practically  unchanged  by  so  slight  a  thing  as  a  forest 
covering. 

341.  Periodic  variations  of  climate.  It  is  a  popular  notion  that  our  climate 
is  changing.  The  winters,  for  example,  are  often  said  to  be  less  severe  than 
when  old  men  were  boys ;  or  the  Gulf  Stream  is  thought  to  shift  its  course  and 
thereby  affect  the  climate  on  our  eastern  coast.  These  errors  arise  in  the  first 
place  from  the  natural  exaggeration  of  past  events,  and  from  the  disposition  to 
forget  facts  of  ordinary  value  and  dwell  on  exceptional  occurrences ;  and  in 
the  second  place  from  a  certain  credulity  regarding  unseen  and  remote  processes. 
While  it  is  well  known  that  the  course  of  the  Gulf  Stream  varies  by  small 
amounts  and  for  short  periods,  it  is  also  well  known  that  its  average  course 
depends  on  long-lived  controls,  such  as  the  shape  of  the  ocean  basin  and  the 
strength  of  the  general  winds;  the  latter  in  turn  depends  on  sunshine,  and 
there  is  no  reason  to  think  that  either  the  ocean  basins  or  the  strength  of 
sunshine  fluctuate  to  the  extent  implied  in  popular  beliefs.  Records  of  rain- 
fall and  temperature  maintained  for  the  longest  series  of  years  do  not  confirm 
the  common  ideas  regarding  our  winters.  The  averages  for  decades  in  the 
early  part  of  the  century  are  essentially  equal  to  those  now  obtained.  If 
slight  differences  appear,  it  is  much  more  likely  that  they  are  due  to  changes 
in  the  instruments  used,  or  in  their  surroundings,  as  by  the  growth  of  trees,  or 
the  building  of  houses,  or  to  changes  in  the  residence  of  the  observers,  than 
that  they  are  due  to  actual  changes  in  terrestrial  or  non-terrestrial  controls  of 
climate.  It  is  true  that  slight  fluctuations  of  rainfall  and  temperature  in 
nearly  eleven  years,  corresponding  to  the  sun-spot  cycle,  have  been  made  out 
at  certain  stations  for  a  moderate  number  of  periods  ;  but  the  fluctuations  have 
not  yet  been  shown  to  be  general,  uniform,  and  persistent.  A  longer  variation 
is  indicated  over  Europe  and  in  certain  other  countries  in  a  period  of  thirty-six 
or  thirty-seven  years,  as  shown  by  Bruckner's  review  of  all  available  records 
of  dry  and  wet  years,  high  and  low  stages  in  rivers,  abundant  and  scanty  crops, 
etc. ;  but  at  least  another  century  will  be  needed  fully  to  confirm  this  result  and 
to  extend  it  over  the  world.  The  middle  dates  of  Bruckner's  periods  of  slightly 
greater  rainfall  and  lower  temperature  are  1671-75,  1696-1700,  1741-45, 
1766-70, 1816-20, 1851-55,  1880;  and  of  less  rainfall  and  higher  temperature, 
1681-85,  1726-30,  1756-60,  1786-90,  1820-30,  1861-65.  One  of  the  most 


CLIMATE.  347 

interesting  indications  of  this  thirty-six  year  period  is  found  in  the  variation 
in  the  lengths  of  Swiss  glaciers;  the  periods  of  extension  be  ing  1760-86, 
1811-22,  1840-55,  1880- ;  and  the  periods  of  retreat  being  1750-67,  1800-12, 
1822-44,  1855-80.  It  is  found  that  the  shorter  glaciers  are  the  first  to  feel 
the  change  in  their  upper  snow  supply,  and  to  lengthen  or  shorten  accordingly ; 
hence  all  the  glaciers  of  the  Alps  are  not  retreating  and  advancing  at  the  same 
time.  Yet  for  a  few  years,  the  longest  and  the  shortest  advance  or  retreat 
together;  thus  from  1815  to  1818,  all  the  Alpine  glaciers  were  advancing; 
from  1822  to  1825,  all  were  retreating  ;  from  1848  to  1850,  all  were  advancing 
again ;  about  1875,  all  were  retreating ;  and  now  another  general  advance  is 
approaching. 

342.  Secular  variation  of  climate.     Ancient  historic  records  around  the 
Mediterranean  Sea  have  been  accepted  by  special  students  of  this  question  as 
indicating  a  general  decrease  of  rainfall  there  in  the  last  three  thousand  years. 
In  northern  Africa,  the  remains  of  cities  imply  a  greater  population  than  can 
now  exist  in  that  desert  region ;   ruins  of  aqueducts  and  irrigating  canals  are 
found  in  districts  where  there  are  now  no  sufficient  streams  to  supply  them; 
ancient  records  mention  the  presence  of  certain  animals  there,  from  which  a 
less  arid  climate  than  the  present  would  be  inferred.    Some  would  ascribe  this 
climatic  change  to  the  ancient  destruction  of  forests ;  but  there  is  no  direct 
evidence  of  the  existence  or  the  destruction  of  forests  along  the  northern  coasts 
of  Africa ;  if  they  once  grew  there  and  were  destroyed  by  man,  it  is  quite  as 
reasonable  to  suppose  that  they  were  then  dwindling  away  and  could  not 
naturally  restore  their  growth  under  an  increasingly  unfavorable  climate,  as  to 
believe  that  the  change  of  climate  was  entirely  due  to  their  destruction. 

343.  Geological  changes  of  climate.     On  passing  from  ancient  historic 
records  back  to  recent  geological  records,  abundant  evidence  is  obtained  of 
climatic  changes.     In  pleistocene  time,  that  division  of  the  geological  scale 
next  preceding  the  present,  the  northwestern  part  of  Europe,  the  northeastern 
part  of  North  America,  and  certain  other  regions  of  less  extent,  were  covered 
with  ice,  much  as  Greenland  is  to-day ;  and  many  interior  basins  where  an 
arid  climate  now  prevails  were  then  flooded  with  broad  lakes.     This  seems 
to  have  happened  not  only  once,  but  repeatedly;   the  records  appearing  to 
be   more    complicated  the  more  closely  they  are  studied.     While  it  is  not 
certain  that  the  lacustrine  conditions  of  the  interior  basins  were  coincident 
with  the  glaciation  of  North  America  and  Europe,  it  is  highly  probable  that 
such  was  the  case  ;  and  that  the  climate  then  prevailing  was  somewhat  colder 
than  now;  thus  increasing  the  length  of  the  cold  season  when  snow  might 
accumulate,  decreasing  the  season  when  it  would  be  melted,  and  diminishing 
the  ratio  of  evaporation  to  rainfall  in  the  now  arid  basins. 


348  ELEMENTARY   METEOROLOGY. 

The  causes  of  the  pleistocene  climatic  variations  have  been  sought  in  a 
change  in  the  altitude  of  land  masses  with  respect  to  sea  level,  in  a  change  in 
the  sun's  heat,  in  a  change  in  the  form  of  the  earth's  orbit,  and  in  various 
other  conditions ;  but  no  general  agreement  is  yet  reached  regarding  them. 
The  changes  in  the  altitude  of  the  land  would  manifestly  induce  an  increase  of 
snowfall ;  but  it  does  not  appear  from  other  evidence  that  the  glaciated  regions 
were  higher  when  the  ice  accumulated  on  them  than  they  are  now.  The  vari- 
ation in  the  form  of  the  earth's  orbit  is  regarded  by  many  students  as  the  most 
effective  control  of  pleistocene  climatic  changes,  and  if  such  is  the  fact,  similar 
changes  should  have  been  of  repeated  occurrence  in  the  geological  past. 
The  variations  referred  to  are  an  increase  in  the  eccentricity  of  the  orbit, 
whereby  the  inequality  of  the  periods  between  the  equinoxes  and  the 
inequality  of  insolation  at  perihelion  and  aphelion  (Sect.  27)  may  be 
increased  ;  and  a  slow  change  in  the  date  of  the  equinoxes,  whereby  first  one 
hemisphere  and  then  the  other  would  have  its  winter  in  aphelion.  At  present, 
the  eccentricity  is  moderate  ;  and  the  northern  or  land  hemisphere  has  its 
winter  in  perihelion.  If  at  a  time  of  great  eccentricity,  the  northern  hemi- 
sphere should  have  its  winter  in  aphelion,  it  is  possible  that  the  accumulation 
of  snow  and  ice  during  so  long  and  severe  a  season  might  not  be  consumed 
in  the  relatively  short  but  hot  summer  ;  thus  snow  and  ice  would  accumulate, 
until  astronomical  changes  brought  back  milder  conditions.  All  this  subject  is 
one  that  still  needs  extended  investigation. 

In  more  remote  geological  ages,  various  climatic  changes  are  indicated. 
Coal  beds  were  formed  in  Greenland ;  forests  grew  in  the  deserts  of  Arizona 
and  Egypt,  ice-borne  boulders  were  carried  into  the  sea  in  which  the  chalk 
of  England  was  formed.  These  and  similar  changes  are  presumably  to  be 
explained  in  great  part  by  changes  in  the  distribution  and  form  of  the  land 
areas,  and  consequent  changes  in  the  course  of  the  winds  and  the  ocean 
currents ;  as  well  as  by  astronomical  changes  of  the  kind  just  mentioned. 


INDEX. 


Abbe,  C.,  319. 

Absorption,   23,   45,   46,    61, 

145. 

Aetinometry,  25. 
Adiabatic  changes  in  tempera- 
ture, 37,  41,  288,  323. 
in  anticyclones,  223,  243, 

246. 

in  cloudy  air,  163-168. 

in    cyclones,    199,    200, 

221-224. 

in  foehn,  238-243. 

African  tornadoes.  255. 
Agriculture,  292,  293. 
Air,  composition,  4. 
expansion,  11. 

—  weight,  143. 
Aitken,  155,  159. 
Alto-cumulus,  169,  177,  178. 
Alto-stratus,  177,  178. 
American      Meteorological 

Journal,  v,  vi,  73,  112,  126, 

204,  213. 
Anemograph,  95. 
Anemometer,  94,  95,  161. 
Anemoscope,  93,  94. 
Aneroid  barometer,  82,  84. 
Anomalies,  thermal,  72,  73. 
Anticyclones,    210,    217-224, 

230,  243-246,  316,  323. 
Aphelion,  19,  69,  348. 
Arched  squall,  255. 
Atmidometry,  146. 
Atmosphere,    absorption,   26, 

45,  46,  51,  145. 

activities,  8. 

circulation,    77,    81,   89, 

110,  115,  153,  204,  206. 

—  composition,  4,  5. 
constitution,  9. 


Atmosphere,  electricity,  265, 

266,  276. 

evolution,  3. 

height,  13. 

instability,  39. 

mass,  10. 

offices,  6. 

origin,  2. 

-  radiation,  26,  173,  174. 

stability,  38. 

waves,  171,  172. 

Aurora,  270,  271. 
Avalanche  blasts,  113. 
Axis  of  cyclone,  226. 

Backing  winds,  215,  227,  229, 

330. 

Balloons,  14,  135,  152,  159. 
Barographs,  84,  85. 
Barometers,  altitude  by,  87. 

—  aneroid,  82,  84. 

corrections  of,  83, 

diurnal   variation,    85, 


185,  312. 

mercurial,  11,  83. 

observation,  86. 

reduction  to  sea  level,  86. 

see  Pressure. 
Barometric  charts,  88,  91,  92. 

—  equator,  91, 100, 120. 
(See  Doldrums  and  Gra- 
dients.) 

surge,  86. 

Berghaus'  Physical  Atlas,  64. 

Bishop's  Ring,  53. 

Black  bulb  thermometer,  61. 

Blanford,  H.  F.,  195. 

Blue  Hill  Observatory,  96, 328. 

cloud    observations,    97, 

121,  179-181,  213,  219. 


Blizzard,  236. 

Bora,  230,  237,  238,  246,  265. 
Brandes,  H.  W.,  187. 
Breezes,  glacial,  138. 
lake,  98,  135. 

—  land  and  sea,  60, 134-137, 
263,  309,  313,  314,  337. 

—  mountain     and    valley, 
137,  138. 

-  tidal,  113. 

"  Brickfielder,"  232. 
Bruckner,  Prof.  E.,  346. 
Buchan,  Prof.  Alex.,  64. 
Buran,  237. 

Calms,  82,  91,  100,  116,  333. 

hi  anticyclones,  218,  230, 

243-246. 

-  in  cyclones,  185, 186, 193, 
202-205,  209,  214. 

—  horse  latitudes,  118,  152. 

-  polar,  116,  205. 
see  Doldrums. 

Calorie,  25,  29. 
Capacity,  142-146,  154. 
Capper,  J.,  187. 
Carbonic  acid,  5,  6,  31. 
Centrifugal  force,  9,  101. 

in  cyclones,  198, 202, 203, 

205,  209,  214. 

—  in  planetary  circulation, 
101,  111,  115,  205. 

in  tornadoes,  277-270. 

1 in  vortices,  109,  110. 

i  Challenger  expedition,  64. 
i  Chambers,  195. 
!  Childs,  W.  H.,  96. 
j  Chinook,  60,  238,  242. 
!  Circulation,  see  Atmosphere, 
convection,  winds. 


350 


INDEX. 


Cirro-cumulus,  170,  177,  178. 
Cirro-stratus,    169,    170,    177, 

178,  185. 
in  thunder  storms,  249, 

251,  255. 
Cirrus,  169-171,  174-178,  316, 

:j:;o,  331. 
in  anticyclones,  219. 

-  in  cyclones,  185, 190,  213, 
214,  228. 

in  thunder  storms,  254. 

Clayton,  H.  H.,  177,  179,  180, 

213,  251,  328. 
Climate,  333-348. 
control  of,  345,  346. 

—  and  habitability,  344,345. 

-  variations,  346-348. 
see  Zones. 

Cloud  bursts,  256. 
Clouds,  146,  330. 

-  altitude,  179-181. 

in  anticyclones,  219,  246. 

classification,  177. 

colors,  160. 

in  cyclones,  185, 186, 190, 

196,  199,  202,  209,  215,  227, 

228. 
dependence  on  dust,  159. 

—  in  foehn,  240. 

—  formation,  40,  132,  159- 
175. 

—  movement,  97,  172,  173, 
182,  213,  219,  227. 

observation,  181. 

—  particles,  159,  160. 

and  rainfall,  285. 

shadows,  52. 

—  in  sirocco,  231. 

—  in  thunder  storms,    41, 
178,248-250,  256,  262-264. 

in   tornadoes,   271,   273, 

L'7'»,  279-281. 

velocity,  97,  180,  182. 

waves,  171,  172. 

Cold  poles,  75,  76. 

Cold  wave,  75,  230,  233-237, 

246,  258,  317,  318,  326. 
Colors  of  clouds,  160. 
sky,  43,  48,  49, 159,  316, 

317,  331. 


Colors  of  sun,  49,  54. 

—  sunrise   and  sunset,  43, 
44,  49-54. 

—  twilight  arch,  44,  51,  250. 
Composition    of    atmosphere, 

4,  5. 
Compression,  heat  caused  by, 

38,  204,  223,  239,  243,  244, 

246. 

and  dew-point,  154. 

Condensation  of  vapor,  154. 

see  Clouds,  dew,  fog,  rain. 
Conduction,   32,    33,    36,   37, 

173,  244. 

Conservation  of  areas,  109. 
Continents,     humidity,     151, 

153. 

—  isotherms,  66. 

—  obstruction     of     winds, 
128,  129. 

—  pressure,  81,  82,  92,  217. 
-  rainfall,  298,  301,  302. 

temperature,  65,  70,  74, 


75. 

-  winds,  82,  97,  122,  128, 
153. 

Convection,    in    anticyclones, 
219-224. 

—  in    atmosphere,   36,   40, 
77-81,  132,  133,  153,  308. 

—  in   clouds,   40,    162-171, 
206. 

—  in      extra-tropical      cy- 
clones,   220-224. 

in  sea  breeze,  136,  206. 

—  in  thunder  storms,  252- 
254,  263. 

—  in  tornadoes,  275-278. 
in  tropical  cyclones,  189, 

190,  194-198,  201,  203,  206- 
208,  215,  216,  221. 
in  water,  35. 

-  in  whirlwinds,  36, 41,201. 
Coronas,  47,  160. 
Cumulo-nimbus,  169, 177, 178, 

249. 

Cumulo-stratus,  174,  177. 
Cumulus  clouds.   l<>.  ]:;•_'.  I'M. 

1 »;-.«.  IT:),  177,  178,  253,  310. 
( 'unvuts  of  oceans,  67-69, 199. 


Cycles,  weather,  :W2. 
Cyclones  and  cyclonic  storms, 

183,  184,  323. 
axis,  226. 

—  centrifugal    force,     198, 


202,  203,  205,  209,  214. 
-clouds,     185,    186,     190, 
196,  199,  202,  209-215,  227, 
228. 

—  convectional  action,  189, 
190,  194-198,  201,  203,  206- 

208,  215,  216,  221. 

—  deflective  force,  197,  198, 
207,  209,  215. 

energy,  200,  223. 

—  extra-tropical,    209-230, 
316-318. 

-  eye,    186,    193,  202-205, 

209,  214. 

—  latent  heat,  199-202,  207, 
216,  223,  225. 

—  law  of  storms,  186-189. 

—  pressure,    185,    202-205, 

209,  210,  227,  278. 

—  progression,  224-227, 323, 
324. 

-rainfall,  185,  186,  190, 
196,  199,  200,  209,  225,  229, 
230,  287,  298,  300. 

-regions,  191-195,  207- 
210. 

—  seasons,    191-195,    207- 

210,  216. 

—  theories,    206-208,    215, 
216,  220-224. 

—  and  thunder  storms,  257- 
261. 

— -  and  tornadoes,  273,  274. 
-tracks,    185,    191,    210, 

211,  225,  22(5. 
-tropical,    184-210,    225, 
2615,   '-'TO. 

-  winds,  185-1 90,  215,  227- 
247. 

-  weather,   185,  215,  227- 
:Mi>,  :!11,  315-319. 

Dampi«-r,  •">!:{. 

Deflective  force  of  earth's  ro- 
tation,  101,  102,  105,  106. 


INDEX. 


351 


Deflective  force,  experiment, 

106-108. 

-  in    cyclones,    197,    108, 

207,  209,  215. 
in  land  and  sea  breezes, 

136. 

in  monsoons,  122,  126. 

— •  in  planetary  circulation, 

115,  116. 

in  tornadoes,  277. 

Deserts,  41,  144,  145, 152, 156; 

232,  241,  255,  291,  304,  344, 

347. 
in  trade  winds,  298,  299, 

336,  338. 

—  in  westerly  winds,  301, 
302. 

Dew,  146,  154-157,  329. 
Dew-point,  146,  148,  154,  163, 

239,  240. 

Diathermance,  23,  145. 
Diffraction,  45,  46,  160. 
Diffusion,  141,  144,  150,  173. 
Doldrums,  100,  117,  276,  336. 
and  cyclones,  185,  192- 

194,  197-199,  215,  225. 

-humidity  of,    144,    150- 

152. 

—  migration   of,    121,  192, 
194,  199. 

-rainfall,   296,   297,  303- 

305. 

weather,  312,  313. 

Dove",  H.  W.,  112,  187,  241. 
Dry  fog,  159. 

Dust,  7,  20,  47,  159,  163,  336. 
Dust  storms,  255,  256. 

Earth,  dimensions,  10. 
internal  heat,  15. 

—  orbit,  19,  348. 

shadow,  51. 

Eclipse  winds,  113. 

Eddy  in  atmosphere,  109, 115, 

216,  221. 

—  in  water,  109,  197. 
see  Vortex. 

Eiffel  tower,  96,  133. 
Electricity,  265,  266,  276. 
Eliot,  J.,  195. 


Equinox,  20,  192. 

Espy,  J.  P.,  133,  162,  241. 

Evaporation,  29,  31,  140,  176, 

338, 

—  amount,  146,  147. 
in  descending  currents, 

167. 
Expansion  of  air,  11,  37-39, 

163,  166. 
Eye  of  storm,  185,  186,  193, 

202-205,  209,  214. 

Ferrel  W.,  iii,  103,  104,  206, 

263,  277. 

Festoon  clouds,  178,  248-250. 
Floods,  292,  293,  304. 
Foehn,  230,  238,  243,  246. 
Fogs,  28,  146,  158,  161,  176, 

178,  246,  330. 
Forests,  294,  295,  345. 
Foucault's  experiment,  102. 
Fracto-cumulus,  178. 
Franklin,  B.,  266,  279. 
Frost,  27,  146,  154,  156,  161, 

333. 

prediction  of,  157. 

protection  from,  158. 

Funnel  cloud,  271,  279-281. 

Galton,  F.,  219. 

Gases,  4,  10. 

General  winds,  129,  136. 

Geosphere,  9. 

Glaciers,  294,  347. 

Glows,  sunrise  and  sunset,  52. 

Gradient,  barometric,  80,  90, 
100,  104,  121,  216,  322,  323. 

in  cyclones,  205. 

— -  deflection  of  wind 

from,  106. 

gravity  on,  90. 

in  planetary  circu- 
lation, 89. 

temperature,  322,  323. 


adiabatic,  38,  40, 

165,  168,  239. 

—  poleward,  65, 69,  71, 
121,  153,  204. 

-  vertical,  27,  37,  162, 
168,  231. 


Gradient,  vertical  temperature, 

in    anticyclones,    221-224, 

243-246. 

in  bora,  238. 

in  cold  waves, 

235,  236. 
in      cyclones, 

221-224. 
in  foehn,  239- 

241. 

in  sirocco,  231. 

in    thunder 

storms,  258-260. 
in    tornadoes, 

275,  276. 

Gravity,  10,  77,  90,  190. 
Greely,  Gen.  A.  W.,  320. 
Gulf  Stream,  68,  69,  212,  230, 

265,  284,  346. 

Hadley,  J.,  102,  103. 

Hail,  154,  250,  252,  254,  261, 

286,  287. 
Halley,  E.,  102. 
Halo,  160,  178,  185. 
Hann,  Prof.  J.,  iii,  64,  220, 

223,  241,  304,  307. 
Harmattan,  232. 
Harrington,  Prof.  M.  W.,  320. 
Harvard  College  Observatory, 

60,  96,  328. 
Haze,  175,  316,  331. 
Hazen,  Gen.  W.  B.,  320. 
Hazen,  Prof.  H.  A.,  149,  257. 
Heat,  by  compression,  38, 204, 

223,  239,  243,  244,  246. 

-  equator,  64,  65,  71,  100, 

120. 

nature  of,  15. 

sources  of,  15. 

of  sun,  18. 

—  unit  of,  25,  29,  141. 
Height  of  atmosphere,  13. 
Hellmann,  Dr.  G.,  133. 
Helm  bar,  172. 
Hills,  temperature  on,  31,  35, 

157,  158. 

Hinrichs,  Prof.  G.,  251,  326. 
Horse  latitudes,  100,  118,  121, 

151,  152,  299,  300. 


352 


INDEX. 


Hot  winds,  133,  232,  233. 
Humidity,  144,  329,  333. 

-  absolute,  145,  151,  235. 
-in    cyclones,    186,    193, 
204,  212. 

distribution,  151,  235. 

-relative,    145,   153,  246, 

250. 
Hurricanes,  94,  186,  188,  192, 

198,  226,  248,  327. 
Mutton,  J.,  174. 
Hydrogen,  3. 
Hydrographic  Office  of  U.S., 

115,  188,  192. 
Hydrosphere,  9. 
Hygrometry,  147. 

Ice  storms,  231,  294. 
Inertia,  9,  105. 
Insolation,  18,  32. 
action  of,  22,  146. 

—  distribution,  19,  20. 
Instability  of  atmosphere,  39, 

258,  259,  276. 
Inversions  of  temperature,  34, 

35,  138,  139,  156,  158. 

-  in  anticyclones,  243-246, 
317. 

• in  siroccos,  231. 

Isobaric  charts,  88,  91,  02. 

Isobaric  lines,  88-91,  212,  322. 

Isobaric  surfaces,  12,  91,  153. 

in  convectional  circula- 
tion, 77,  78,  308. 

in  cyclones,  226. 

Isothermal  charts,  63,  64,  69, 
334. 

-  lines,  64-66,  69,  212, 213, 
:J22. 

Junghuhn,  F.  W.,  313. 
Jupiter,  atmosphere,  3. 
winds,  119,  120. 

Khamsin,  232. 

Koppen,   Dr.    W.,    126,    133, 

252. 
Krakatoa,  53,  85,  119. 

Labrador  current,  68,  69. 
Lake  breeze,  98,  136. 


Land,  absorption  by,  30. 
range  of  temperature,  31, 

32. 
Land  and  sea  breezes,  60, 134- 

136,  263,  309,  313,  314. 
Landslide  blast,  113. 
Langley,  Prof.  S.  P., 25, 26, 49. 
Latent  heat,  29,  140,  141,  155. 
in    ascending    currents, 

165,  166. 

—  in  cyclones,  199-202, 207, 
216,  223,  225. 

—  in  foehn,  240,  241. 

in  thunder  storms,  263. 

in  tornadoes,  275. 

Law  of  storms,  186-189. 
Leste,  232. 

Leveche,  232. 

Leverrier,  U.  J.  J.,  319,  328. 

Lick  Observatory,  96. 

Light,  24,  45. 

Lightning,  186,  248,  250,  254, 

266-269. 

Lightning  rods,  269. 
Literal  winds,  134,  136. 
Looming,  55. 
Loomis,  Prof.  E.,  210, 225,329. 

Meldrum,  C.,  194,  195. 
Meteors,  13,  14. 
Mistral,  236. 
Moisture,  140. 
Monsoons,  Australian,  127. 
continental,  123. 

-Indian,  126, 127, 152, 313, 

336. 

terrestrial,  120,  121. 

Moon,  332. 

Mountains,  50,  99,  137,  161, 

172. 

barriers,  161. 

breezes,  137,  138. 

-  climate,  337,  338,  343. 

—  frost-work,  161. 

observatories,  90,  07. 

rainfall,  287-289. 

shadows,  52. 

thunder  storms,  266. 

Mt.  Washington,  96,  133,  161, 

227,  231,  294. 


Myer,  Gen.  A.  W.,  319. 
New  England  Meteorological 

Society,  59. 
Neutral  plane,  79. 
Newton,  Prof.  H.  A.,  14. 
Nimbus,  177,  178,  185. 
Nitrogen,  4,  5,  7. 
Noah's  ark,  178. 
Norther,  236,  326. 
Northwesters,  243,  255. 

Observations,  barometer,  86. 
clouds,  181,  182. 


rainfall.  289,  290. 

—  temperature,  61,  62. 
thunder  storms,  250-252. 

tornadoes,  282,  283. 

-  weather,  311,  328,  334. 
—  wind,  93,  97,  98. 

Observatories,  328. 

-Blue   Hill,    96,    97,    121, 
179-181,  213,  219. 

-  Harvard  College,  60,  96, 

-  Lick,  96.  [328. 
mountain,  96,  97. 

-  Pikes  Peak,  60,  96,  119, 
246. 

Sonnblick,  97,  223. 

Orbit  of  earth,  19,  42,  348. 
Oxygen,  4,  5. 
Ozone,  5. 

Pampero,  237,  256. 
Pericyclonic    ring,    185,   190, 

203,  205,  210,  21!). 
Perihelion,  19,  22,  69,  34d. 
Piddington,  H.,  194. 
Pikes  Peak,    52,  60,   96,   98, 

119,  245. 

Planetary  winds,  114,  204, 207. 
Poey,  195. 
Polar  bands,  178. 
Polarization,  48. 
Pressure  of  atmosphere,  10, 11, 

13,  92. 

—  charts  of,  88. 

on  continents,  81,  82,  92, 

217. 

in  convectional  circula- 
tion, 77-79. 


INDEX. 


353 


Pressure    of    atmosphere,    in  I 

cyclones,  185,  202-205,  209, 

210,  227,  278. 

distribution  of,  88. 

diurnal,  85,  185,  312. 

equatorial,  82,   88,    100, 

120. 

measurement,  82. 

of  ocean,  12. 

polar,  82,  93,  101,   103, 

110,  111,  129,  130,  153,  204, 

205,  207,  278. 
in  thunder  storms,  250, 

263,  279. 

. in  tornadoes,  278. 

tropical,  88,  91,  93,  100, 

101,  111,  114,  121,  204,  205, 

339. 
Prevailing  westerly  winds,  99, 

101,  116,  118,  128,  227. 
—  deserts,  301,  302. 

rainfall,  300-303. 

storms,    210,    216,    221, 

224,  225. 

Psychrometer,  148,  149. 
Purga,  237. 

Radient  energy,  17,  42. 
Radiation,  from  clouds,  174. 

air,  26,  173,  174,  219. 

earth,   25,   26,   31,    145, 

157,  158,  176. 

sun,  see  Insolation. 

Rain,  161,  285-309,  333. 

and  agriculture,  292,  293. 

and  altitude,  288,  ^89. 

artificial,  295,  345. 

causes,  285,  287,  296. 

on  continents,  298,  301, 

302. 
in  cyclones,  185, 186, 190, 

196,  199,  200,  209,  225,  229, 

230,  287,  298,  300. 
in    doldrums,    117,   296, 

297. 

—  drops,  285. 
equatorial,  296,  297,  300, 

336,  338. 

and  forests,  294,  295,  345. 

horse  latitudes,  299,  300. 

measurement,  289,  290. 


Rain  records,  290-292,  333. 

—  sub-equatorial,  304,  305, 
337. 

—  sub-tropical,  306,  307. 
in  thunder  storms,  250, 

254,  267. 

—  in  trade  winds,  297,  298. 
in  westerly  winds,  300- 

303. 

Rainbow,  250,  296,  331. 
Rain  gauge,  289,  290. 
Range  of  temperature,  in  air, 

28. 
annual,  74. 

-  diurnal,  27,  30,  42,  134, 

155,  319. 

—  in  land,  31,  32. 

in  ocean,  29. 

Rayleigh,  Lord,  48. 
Redfield,  W.  C.,  187, 192,  227. 
Reflection,  22,  45. 
Refraction,  50,  160. 

Reid,  Sir  W.,  192. 
''Roaring  forties,"  99. 
Rotation  of  earth,  10,  277. 

see  Deflection. 
Rotch,  A.  L.,  96,  328. 
"Roundabouts,"  135. 

Saturation,  142,  143,  146, 163. 
Scattering  of  light,  46,  48,  50, 

51. 

Schiick,  A.,  195. 
Scud,  161,  190. 
Sea  breeze,  60,  134-136,  263, 

309,  313,  314,  337. 
Seemann,  136 
Serein,  288. 
Shadow  of  clouds,  165. 

—  earth,  44,  51,  52. 

mountains,  52. 

Silver  thaw,  231,  294. 
Simoom,  233,  255. 

Sirocco,  230-233, 236, 246, 258, 

273,274,  316. 
Sky,  colors  of,  43,  48,  49, 159, 

316,  317,  331. 
Sling  thermometer,  57. 
Smithsonian  Institution,  62,87. 
Smoke,  27,  46,  158,  159. 


Snow,  154,  160,  178,  286,  290, 

293,  294,  303,  317,  333. 
Solano,  232. 
Solar  constant,  25. 
Solstices,  19,  22,  121. 
Sonnblick,  97,  223. 
Spectrum,  25,  50. 
Squall,  233,  248-252,  255,  262- 

264,  273. 

Squall  cloud,  249,  262-264. 
St.  Elmo's  fire,  268. 
State  Weather  Services,   67, 

327,  328. 
Sub-equatorial  belt,  122,  304, 

305,  336,  337. 
Sub-tropical  belt,  122, 306, 307, 

338. 

Sun,  15,  18?  19,  49,  54,  332. 
Sunset  and  sunrise'  colors,  43, 

49,  50,  52. 

of  1883-84,  53,  54. 

Sunshine,  182,  333,  334. 
Sunstrokes,  316. 
Surge,  86. 
Synoptic  maps,  187. 

Temperature,    see    Adiabatic 

changes. 
in  anticyclones,  218,  220, 

224,  243-246,  316,  323. 
in  cyclones,  186, 193, 204, 

212,  220,  224-226. 

distribution,  64. 

inversions,    in    anticy- 
clones, 243-246,  317. 
nocturnal,   34,   35, 

138,  139,  156,  158. 

in  siroccos,  231. 

mean,  62,  63,  336. 

observations,  61,  62. 

polar,  75. 

poleward  gradients,   66, 

69,  71,  153,  204. 

range,  in  air,  28. 

annual,  74. 

in  anticyclones,  243. 

i  n    cyclones,    319, 

333. 

-  diurnal,  27,  30,  42, 

134,  155,  319. 


354 


INDEX. 


Temperature,  range,  in  land, 

31,  32. 

in  ocean,  29. 

records,  61,  333. 

sea  breeze,  60,  134. 

sirocco,  230-232. 

in  thunder  storms,  250, 

251. 

underground,  32,  33,  333. 

vertical  gradient,  27,  37, 

162,  168,  231. 
Terrestrial  winds,  120. 
Thermographs,  58,  59. 
Thermometers,  56. 

black  bulb,  61. 

exposure,  57. 

maximum  and  minimum, 

60. 

sling,  57. 

Thunder,  249,  254,  268,  269. 
Thunder    storms,     167,    169, 

183,  184,  248-269,  296,  297, 

316. 

and  cyclones,  257-261. 

clouds,  41,  178,  248-250, 

256,  262-264. 

convection,  252-261. 

distribution,  254-256. 

— - —  hi  doldrums,  117. 

and  hail,  287. 

on  mountains,  256. 

nocturnal,  264. 

pressure    in,    250,    263, 

279. 
progression,  250-252, 261, 

262. 
squalls,  233,  248-252, 

255,  262-264,  273. 
and  tornadoes,  273,  275, 

279. 
Tornadoes,  183,  184,  271-284. 

African,  255. 

clouds,    271,    273,    275, 

279-281. 

and  cyclones,  273,  274. 

distribution,  273. 

pressure  in,  278,  279. 

progression,  281. 

vortex,  276,  277-279. 

Tracy,  C.,  103. 


Trade  winds,  99, 105, 120,  121, 
128,  176,  199,  334. 

—  clouds,  170-173. 

—  deserts,   298,    299,    336, 
338. 

—  humidity,  151,  152. 
rains,  297,  298. 

—  weather,  311,  312. 
Transmission,  23,  46,  51. 
Tropics,  meteorological,  88. 
Twilight,  13,  20. 

-  arch,  44,  51,  250. 
Typhoon,  193. 

Unit  of  heat,  25,  29,  141. 

Valley  breezes,  137,  138. 

—  fogs,  158. 

—  temperature,  31,  35,  157. 

and  thunder  storms,  202. 

Vapor,  see  Water  vapor. 
Veering  winds,  135,  227,  229, 

330. 

Vettin,  180. 
Volcanic  eruptions,  53,  54. 

winds,  113. 

Vortex,    109,    110,   198,   203, 

213. 
in  tornadoes,   273,   276, 

283. 

Warm  wave,  230-233. 
Water,  30. 

Waterspout,  272,  283  284. 
Water  vapor,  absorption,  32, 
145. 

in  cyclones,  200. 

distribution,  150. 

effect  on  winds,  153. 

—  pressure,  152. 

-  weight,  141,  143. 
Waves    in    atmosphere,    171, 

172. 

-  in  storms,  185,  186,  189, 
192. 

Weather,  310-332. 

—  in  anticyclones,  218,  317, 
319. 

cycles,  332. 

—  in    cyclones,    185,    215, 
227-230,  311,  315-319. 


Weather,  diurnal,  311,  316. 

elements,  310. 

frigid,  318. 

-  maps,  183,  321-324,  328, 
329. 

observations,    311,    318, 

320,  #54. 

-  prediction,      324,      325, 
332. 

—  proverbs,  329-331. 

—  signals,  325-327. 

-  temperate,  314,  318. 

-  torrid,  312,  314. 

-  types,  323. 

Weather  Bureau,  57,  62,  233, 

307,  319. 

Weather  Services,  328,  329. 
Wells,  C.  W.,  155. 
Whirlwinds,  36,  41,  169,  201, 

202,  276. 
Winds,  in  anticyclones,  219. 

Arctic,  129. 

avalanche,  113. 

backing,     215,      227, 

229. 
belts,  117,  119. 

—  circumpolar,    101,     110, 
115,  129,  204,  216,  217,  277, 
278. 

classification,  112. 

continental,     122,     128, 

153. 

convectional,  77,  100.    ' 

see  Cyclones. 

deflection  of,  106. 

diurnal    variation,    132, 

133,  162,  236. 
eclipse,  113. 

—  force  of,  94. 

—  general,    129,    136,   137, 
211),  224. 

—  "hot  winds,"  133. 

—  land  slide,  113. 

-  literal,  1M,  137. 

-  migration  i.f,  120,  121. 

—  mountain  ami  valley,  137, 
138. 

observation,  93,  97,  98. 

-  planetary,  114,  115,  204, 
207. 


INDEX. 


355 


Winds,  prevailing  westerly,  90, 
IB1,  116,  118,  128,  227,  296, 
301,  302,  334. 

terrestrial,  120,  216. 

—  tidal,  113. 

see  Trade  winds. 

upper,    105,     119,    213, 


216. 

—  veering,    135,  215,    227, 
229,  330. 


Winds,  velocity,  94,  96. 
volcanic,  113. 

Zonda,  232. 
Zones,  22,  334. 

frigid,  climate,  344. 

storms,  184. 

—  weather,  318. 

temperate,  climate,  339- 

343. 


Zones,  temperate,  coasts,  341- 
343. 

—  storms,  184,208-224, 

weather,  230. 

torrid,  156,  158,  165, 190, 

210,  224,  225. 
climate,  335-338. 

—  storms,  184,  198. 
weather,  230,  312- 

314. 


Davis'  Elementary  Physical  Geography 


By    WILLIAM    MORRIS    DAVIS 

Author  of  Dams'  "Physical  Geography  "  and  Professor  of  Geology  in 


Harvard  University 


i2mo.    Cloth,    viii  +  401  pages  +  6  color  charts  +  16  pages  of  maps. 
Illustrated.    List  price,  $1.25. 


T 


HE  "  Elementary  Physical  Geography  "  is  a  new  book 
by  Professor  W.  M.  Davis,  the  first  authority  in  the 
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earlier  work,  "  Physical  Geography,"  but  is  much  less  difficult 
and  is  admirably  adapted  for  use  with  younger  pupils. 

The  plan  of  this  volume,  like  that  of  its  predecessor,  is  to  give 
the  problems  of  physical  geography  a  rational  treatment.  The 
object  of  this  method  is  not  simply  to  explain  physiographic 
facts,  but  to  increase  the  appreciation  of  the  facts  themselves  by 
associating  them  with  their  causes  and  their  consequences.  This 
relation  is  not  presented  merely  as  an  afterthought  in  a  detached 
chapter  at  the  end  of  the  book ;  it  accompanies  the  presentation 
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The  chapter  on  the  Atmosphere  has  been  considerably  expanded, 
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THE 

Harvard  Geographical  Models 


DESIGNED    BY 


WILLIAM  M.   DAVIS, 

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use  of  models  or  reliefs  in  teaching  geography  has 
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THE  ELEMENTS  OF  GEOLOGY 

By  WILLIAM  HARMON  NORTON 

Professor  of  Geology  in  Cornell  College,  Mt.  Vernon,  Iowa 


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THE 
LAKES  OF  NORTH  AMERICA 

By  ISRAEL  C.  RUSSELL 
Professor  of  Geology  in  the  University  of  Michigan 

8vo.    Cloth.    xi+ 125  pages.    Illustrated. 

RECENT  advances  have  made  physical  geography  almost  a  new 
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GLACIERS  OF  NORTH  AMERICA 

By  ISRAEL  C.  RUSSELL,  Professor  of  Geology  in  the  University  of  Michigan 
Author  of  "  Lakes  of  North  America  " 

8vo.    Cloth.    x+ 210 pages.    Illustrated. 

RECENT  explorations  have  shown  that  North  America  contains 
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represented.     In    the   study  of    the   glaciers    of   North    America,  and 
especially  of  those  in  Alaska,  Professor  Russell  has  taken  an  active 
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will  make  it  of  interest  to  the  general  reader. 


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